Give the structural formula for the compound represented by this line-angle formula:. As noted in Table 1. There are 3 pentanes, 5 hexanes, 9 heptanes, and 18 octanes. It would be difficult to assign unique individual names that we could remember. Some of the names we used earlier, such as isobutane, isopentane, and neopentane, do not follow these rules and are called common names.
A stem name Table 1. Atoms or groups attached to this carbon chain, called substituents , are then named, with their positions indicated by numbers. For now, we will consider only those substituents called alkyl groups.
An alkyl group is a group of atoms that results when one hydrogen atom is removed from an alkane. The group is named by replacing the -ane suffix of the parent hydrocarbon with -yl.
For example, the CH 3 group derived from methane CH 4 results from subtracting one hydrogen atom and is called a methyl group. The alkyl groups we will use most frequently are listed in Table 1. Alkyl groups are not independent molecules; they are parts of molecules that we consider as a unit to name compounds systematically. We will introduce the other three where appropriate. Name alkanes according to the LCC of carbon atoms in the molecule rather than the total number of carbon atoms.
This LCC, considered the parent chain, determines the base name, to which we add the suffix -ane to indicate that the molecule is an alkane. If the hydrocarbon is branched, number the carbon atoms of the LCC.
Numbers are assigned in the direction that gives the lowest numbers to the carbon atoms with attached substituents. Hyphens are used to separate numbers from the names of substituents; commas separate numbers from each other. Place the names of the substituent groups in alphabetical order before the name of the parent compound.
If the same alkyl group appears more than once, the numbers of all the carbon atoms to which it is attached are expressed. If the same group appears more than once on the same carbon atom, the number of that carbon atom is repeated as many times as the group appears.
Moreover, the number of identical groups is indicated by the Greek prefixes di -, tri -, tetra -, and so on. These prefixes are not considered in determining the alphabetical order of the substituents. For example, ethyl is listed before dimethyl; the di- is simply ignored. The last alkyl group named is prefixed to the name of the parent alkane to form one word.
When these rules are followed, every unique compound receives its own exclusive name. The rules enable us to not only name a compound from a given structure but also draw a structure from a given name. Then add the groups at their proper positions. The name indicates two methyl CH 3 groups, one on the second carbon atom and one on the third.
Finally, fill in all the hydrogen atoms, keeping in mind that each carbon atom must have four bonds. Filling in all the hydrogen atoms gives the following condensed structural formulas:. Note that the bonds dashes can be shown or not; sometimes they are needed for spacing.
What is a CH 3 group called when it is attached to a chain of carbon atoms—a substituent or a functional group? Briefly identify the important distinctions between an alkane and an alkyl group. What is a substituent? An alkane is a molecule; an alkyl group is not an independent molecule but rather a part of a molecule that we consider as a unit. Common names are widely used but not very systematic; IUPAC names identify a parent compound and name other groups as substituents.
Because alkanes have relatively predictable physical properties and undergo relatively few chemical reactions other than combustion, they serve as a basis of comparison for the properties of many other organic compound families. Because alkane molecules are nonpolar, they are insoluble in water, which is a polar solvent, but are soluble in nonpolar and slightly polar solvents. Consequently, alkanes themselves are commonly used as solvents for organic substances of low polarity, such as fats, oils, and waxes.
Nearly all alkanes have densities less than 1. These properties explain why oil and grease do not mix with water but rather float on its surface.
Gas densities are at 1 atm pressure. The leak was a mile below the surface, making it difficult to estimate the size of the spill. One liter of oil can create a slick 2. Natural gas is composed chiefly of methane, which has a density of about 0. The density of air is about 1. Because natural gas is less dense than air, it rises.
When a natural-gas leak is detected and shut off in a room, the gas can be removed by opening an upper window. On the other hand, bottled gas can be either propane density 1. Both are much heavier than air density 1. If bottled gas escapes into a building, it collects near the floor.
This presents a much more serious fire hazard than a natural-gas leak because it is more difficult to rid the room of the heavier gas.
As shown in Table 1. This general rule holds true for the straight-chain homologs of all organic compound families. Larger molecules have greater surface areas and consequently interact more strongly; more energy is therefore required to separate them. For a given molar mass, the boiling points of alkanes are relatively low because these nonpolar molecules have only weak dispersion forces to hold them together in the liquid state. An understanding of the physical properties of the alkanes is important in that petroleum and natural gas and the many products derived from them—gasoline, bottled gas, solvents, plastics, and more—are composed primarily of alkanes.
This understanding is also vital because it is the basis for describing the properties of other organic and biological compound families. For example, large portions of the structures of lipids consist of nonpolar alkyl groups.
Lipids include the dietary fats and fatlike compounds called phospholipids and sphingolipids that serve as structural components of living tissues. For more information about lipids, see Chapter 7 "Lipids". These compounds have both polar and nonpolar groups, enabling them to bridge the gap between water-soluble and water-insoluble phases. This characteristic is essential for the selective permeability of cell membranes.
Tripalmitin a , a typical fat molecule, has long hydrocarbon chains typical of most lipids. Compare these chains to hexadecane b , an alkane with 16 carbon atoms. Without referring to a table, predict which has a higher boiling point—hexane or octane. If 25 mL of hexane were added to mL of water in a beaker, which of the following would you expect to happen?
Hexane would not dissolve in water and would sink to the bottom of the container. Without referring to a table or other reference, predict which member of each pair has the higher boiling point.
Alkane molecules are nonpolar and therefore generally do not react with ionic compounds such as most laboratory acids, bases, oxidizing agents, or reducing agents. Consider butane as an example:. Neither positive ions nor negative ions are attracted to a nonpolar molecule.
Two important reactions that the alkanes do undergo are combustion and halogenation. Nothing happens when alkanes are merely mixed with oxygen O 2 at room temperature, but when a flame or spark provides the activation energy, a highly exothermic combustion reaction proceeds vigorously. For methane CH 4 , the reaction is as follows:.
If the reactants are adequately mixed and there is sufficient oxygen, the only products are carbon dioxide CO 2 , water H 2 O , and heat—heat for cooking foods, heating homes, and drying clothes.
Because conditions are rarely ideal, however, other products are frequently formed. When the oxygen supply is limited, carbon monoxide CO is a by-product:. This reaction is responsible for dozens of deaths each year from unventilated or improperly adjusted gas heaters. Similar reactions with similar results occur with kerosene heaters. Alkanes also react with the halogens chlorine Cl 2 and bromine Br 2 in the presence of ultraviolet light or at high temperatures to yield chlorinated and brominated alkanes.
Fluorine F 2 , the lightest halogen, combines explosively with most hydrocarbons. Iodine I 2 is relatively unreactive. Fluorinated and iodinated alkanes are produced by indirect methods. We will discuss the names and uses of halogenated hydrocarbons in Section 1.
Which halogen reacts most readily with alkanes? Which reacts least readily? Alkanes do not react with many common chemicals. Why do alkanes usually not react with ionic compounds such as most laboratory acids, bases, oxidizing agents, or reducing agents? Write an equation for the complete combustion of methane CH 4 , the main component of natural gas. Name some substances other than oxygen that react readily with alkanes.
Many organic compounds are closely related to the alkanes. As we noted in Section 1. Even more closely related are the cycloalkanes, compounds in which the carbon atoms are joined in a ring, or cyclic fashion.
The reactions of alkanes with halogens produce halogenated hydrocarbons , compounds in which one or more hydrogen atoms of a hydrocarbon have been replaced by halogen atoms:. The replacement of only one hydrogen atom gives an alkyl halide or haloalkane. The common names of alkyl halides consist of two parts: the name of the alkyl group plus the stem of the name of the halogen, with the ending -ide.
The prefixes are fluoro -, chloro -, bromo-, and iodo -. Alkyl halides with simple alkyl groups one to four carbon atoms are often called by common names. A wide variety of interesting and often useful compounds have one or more halogen atoms per molecule. For example, methane CH 4 can react with chlorine Cl 2 , replacing one, two, three, or all four hydrogen atoms with Cl atoms.
Once widely used in consumer products, many chlorinated hydrocarbons are suspected carcinogens cancer-causing substances and also are known to cause severe liver damage.
An example is carbon tetrachloride CCl 4 , once used as a dry-cleaning solvent and in fire extinguishers but no longer recommended for either use.
Even in small amounts, its vapor can cause serious illness if exposure is prolonged. Moreover, it reacts with water at high temperatures to form deadly phosgene COCl 2 gas, which makes the use of CCl 4 in fire extinguishers particularly dangerous. Ethyl chloride, in contrast, is used as an external local anesthetic. When sprayed on the skin, it evaporates quickly, cooling the area enough to make it insensitive to pain.
It can also be used as an emergency general anesthetic. Bromine-containing compounds are widely used in fire extinguishers and as fire retardants on clothing and other materials. Because they too are toxic and have adverse effects on the environment, scientists are engaged in designing safer substitutes for them, as for many other halogenated compounds. Alkanes substituted with both fluorine F and chlorine Cl atoms have been used as the dispersing gases in aerosol cans, as foaming agents for plastics, and as refrigerants.
Two of the best known of these chlorofluorocarbons CFCs are listed in Table 1. Chlorofluorocarbons contribute to the greenhouse effect in the lower atmosphere. They also diffuse into the stratosphere, where they are broken down by ultraviolet UV radiation to release Cl atoms.
These in turn break down the ozone O 3 molecules that protect Earth from harmful UV radiation. Worldwide action has reduced the use of CFCs and related compounds.
The CFCs and other Cl- or bromine Br -containing ozone-destroying compounds are being replaced with more benign substances. HCFC molecules break down more readily in the troposphere, and fewer ozone-destroying molecules reach the stratosphere. They occur mainly over Antarctica from late August through early October and fill in about mid-November. Ozone depletion has also been noted over the Arctic regions. The largest ozone hole ever observed occurred on 24 September Hint: you must use a number to indicate the location of each substituent F atom.
Write the condensed structural formula for each compound. Write the condensed structural formulas for the two isomers that have the molecular formula C 3 H 7 Br. Write the condensed structural formulas for the four isomers that have the molecular formula C 4 H 9 Br. What is a CFC? How are CFCs involved in the destruction of the ozone layer? Explain why each compound is less destructive to the ozone layer than are CFCs. The hydrocarbons we have encountered so far have been composed of molecules with open-ended chains of carbon atoms.
When a chain contains three or more carbon atoms, the atoms can join to form ring or cyclic structures. The simplest of these cyclic hydrocarbons has the formula C 3 H 6. Each carbon atom has two hydrogen atoms attached Figure 1. It is also a potent, quick-acting anesthetic with few undesirable side effects in the body. It is no longer used in surgery, however, because it forms explosive mixtures with air at nearly all concentrations.
The cycloalkanes —cyclic hydrocarbons with only single bonds—are named by adding the prefix cyclo- to the name of the open-chain compound having the same number of carbon atoms as there are in the ring.
Thus the name for the cyclic compound C 4 H 8 is cyclobutane. The carbon atoms in cyclic compounds can be represented by line-angle formulas that result in regular geometric figures.
Keep in mind, however, that each corner of the geometric figure represents a carbon atom plus as many hydrogen atoms as needed to give each carbon atom four bonds. Some cyclic compounds have substituent groups attached. Example 5 interprets the name of a cycloalkane with a single substituent group.
The name cyclopentane indicates a cyclic cyclo alkane with five pent- carbon atoms. It can be represented as a pentagon. The name methylcyclobutane indicates a cyclic alkane with four but- carbon atoms in the cyclic part. It can be represented as a square with a CH 3 group attached. The properties of cyclic hydrocarbons are generally quite similar to those of the corresponding open-chain compounds. So cycloalkanes with the exception of cyclopropane, which has a highly strained ring act very much like noncyclic alkanes.
Cyclic structures containing five or six carbon atoms, such as cyclopentane and cyclohexane, are particularly stable.
We will see in Chapter 6 "Carbohydrates" that some carbohydrates sugars form five- or six-membered rings in solution. This strain is readily evident when you try to build a ball-and-stick model of cyclopropane; see Figure 1. Cyclopentane and cyclohexane rings have little strain because the C—C—C angles are near the preferred angles. Cycloalkyl groups can be derived from cycloalkanes in the same way that alkyl groups are derived from alkanes. These groups are named as cyclopropyl, cyclobutyl, and so on.
Name each cycloalkyl halide. As with alkyl derivatives, monosubstituted derivatives need no number to indicate the position of the halogen. To name disubstituted derivatives, the carbon atoms are numbered starting at the position of one substituent C1 and proceeding to the second substituted atom by the shortest route. Name each compound. To ensure that you understand the material in this unit, you should review the meanings of the following bold terms in the summary and ask yourself how they relate to the topics in the unit.
Organic chemistry is the chemistry of carbon compounds, and inorganic chemistry is the chemistry of all the other elements. Carbon atoms can form stable covalent bonds with other carbon atoms and with atoms of other elements, and this property allows the formation the tens of millions of organic compounds. Hydrocarbons contain only hydrogen and carbon atoms. Hydrocarbons in which each carbon atom is bonded to four other atoms are called alkanes or saturated hydrocarbons.
Any given alkane differs from the next one in a series by a CH 2 unit. Any family of compounds in which adjacent members differ from each other by a definite factor is called a homologous series. Carbon atoms in alkanes can form straight chains or branched chains. Two or more compounds having the same molecular formula but different structural formulas are isomers of each other.
There are no isomeric forms for the three smallest alkanes; beginning with C 4 H 10 , all other alkanes have isomeric forms. A structural formula shows all the carbon and hydrogen atoms and how they are attached to one another. A condensed structural formula shows the hydrogen atoms right next to the carbon atoms to which they are attached. A line-angle formula is a formula in which carbon atoms are implied at the corners and ends of lines.
Each carbon atom is understood to be attached to enough hydrogen atoms to give each carbon atom four bonds. An alkyl group is a unit formed by removing one hydrogen atom from an alkane. The physical properties of alkanes reflect the fact that alkane molecules are nonpolar. Alkanes are insoluble in water and less dense than water. Alkanes are generally unreactive toward laboratory acids, bases, oxidizing agents, and reducing agents. They do burn undergo combustion reactions.
Alkanes react with halogens by substituting one or more halogen atoms for hydrogen atoms to form halogenated hydrocarbons. An alkyl halide haloalkane is a compound resulting from the replacement of a hydrogen atom of an alkane with a halogen atom.
Cycloalkanes are hydrocarbons whose molecules are closed rings rather than straight or branched chains. A cyclic hydrocarbon is a hydrocarbon with a ring of carbon atoms. It ignites readily and burns readily. Petrovskii and V. Bregadze, Dalton Trans. To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.
If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. Read more about how to correctly acknowledge RSC content. Fetching data from CrossRef. This may take some time to load. Loading related content. Jump to main content. Note that the fatty acids shown in Figure 8. When the fatty acids from the TAG shown in Figure 8. Thus, monounsaturated and polyunsaturated fats cannot stack together as easily and do not have as many intermolecular attractive forces when compared with saturated fats.
As a result, they have lower melting points and boiling points and tend to be liquids at room temperature. It has been shown that the reduction or replacement of saturated fats with mono- and polyunsaturated fats in the diet, helps to reduce levels of the low-density-lipoprotein LDL form of cholesterol, which is a risk factor for coronary heart disease. Trans-fats, on the other hand, contain double bonds that are in the trans conformation. Thus, the shape of the fatty acids is linear, similar to saturated fats.
Trans fats also have similar melting and boiling points when compared with saturated fats. However, unlike saturated fats, trans-fats are not commonly found in nature and have negative health impacts. Trans-fats occur mainly as a by-product in food processing mainly the hydrogenation process to create margarines and shortening or during cooking, especially deep fat frying.
In fact, many fast food establishments use trans fats in their deep fat frying process, as trans fats can be used many times before needing to be replaced.
Consumption of trans fats raise LDL cholesterol levels in the body the bad cholesterol that is associated with coronary heart disease and tend to lower high density lipoprotein HDL cholesterol the good cholesterol within the body. Trans fat consumption increases the risk for heart disease and stroke, and for the development of type II diabetes.
The risk has been so highly correlated that many countries have banned the use of trans fats, including Norway, Sweden, Austria and Switzerland.
This measure is estimated to prevent 20, heart attacks and 7, deaths per year. Which compounds can exist as cis-trans geometric isomers? Draw them. All four structures have a double bond and thus meet rule 1 for cis-trans isomerism.
This compound meets rule 2; it has two nonidentical groups on each carbon atom H and Cl on one and H and Br on the other. It exists as both cis and trans isomers:. This compound meets rule 2; it has two nonidentical groups on each carbon atom and exists as both cis and trans isomers:. Which compounds can exist as cis-trans isomers? What are cis-trans geometric isomers? What two types of compounds can exhibit cis-trans isomerism? Classify each compound as a cis isomer, a trans isomer, or neither.
Cis-trans isomers are compounds that have different configurations groups permanently in different places in space because of the presence of a rigid structure in their molecule. Alkenes and cyclic compounds can exhibit cis-trans isomerism. The situation becomes more complex when there are 4 different groups attached to the carbon atoms involved in the formation of the double bond.
The cis-trans naming system cannot be used in this case, because there is no reference to which groups are being described by the nomenclature.
For example, in the molecule below, you could say that the chlorine is trans to the bromine group, or you could say the chlorine is cis to the methyl CH 3 group. Thus, simply writing cis or trans in this case does not clearly delineate the spatial orientation of the groups in relation to the double bond. Naming the different stereoisomers formed in this situation, requires knowledge of the priority rules. Recall from chapter 5 that in the Cahn-Ingold-Prelog CIP priority system, the groups that are attached to the chiral carbon are given priority based on their atomic number Z.
Atoms with higher atomic number more protons are given higher priority i. E comes from the German word entgegen, or opposite. Thus, when the higher priority groups are on the opposite side of the double bond, the bond is said to be in the E conformation. Z , on the other hand, comes from the German word zusammen, or together. Thus, when the higher priority groups are on the same side of the double bond, the bond is said to be in the Z conformation.
As we saw in Chapter 7, small alkanes can be formed by the process of thermal cracking. This process also produces alkenes and alkynes. In comparison to alkanes, alkenes and alkynes are much more reactive.
In fact, alkenes serve as the starting point for the synthesis of many drugs, explosives, paints, plastics and pesticides. Since combustion reactions were covered heavily in Chapter 7, and combustion reactions with alkenes are not significantly different than combustion reactions with alkanes, this section will focus on the later four reaction types. Most reactions that occur with alkenes are addition reactions.
As the name implies, during an addition reaction a compound is added to the molecule across the double bond. The result is loss of the double bond or alkene structure , and the formation of the alkane structure.
The reaction mechanism of a reaction describes how the electrons move between molecules to create the chemical reaction. Note that in reaction mechanism diagrams, as shown in Figure 8. The reaction mechanism for a generic alkene addition equation using the molecule X-Y is shown below:.
Reaction mechanism of a generic addition reaction. In this reaction, an electron from the carbon-carbon double bond of the alkene attacks an incoming molecule XY causing the breakage of the carbon-carbon double bond lefthand diagram and formation of a new bond between one of the alkene carbons and molecule X.
The original electron from X that was participating in the shared bond with Y, is donated to Y causing the breakage of the X-Y bond. In the intermediate state middle diagram , the alkene is carrying a positively charged carbon ion, called a carbocation , and Y is in a negatively charged anion state.
The negative anion is attracted to the positively charged carbocation and donates the two electrons to form the C-Y bond and complete the product of the addition reaction righthand diagram.
Addition reactions convert an alkene into an alkane by adding a molecule across the double bond. There are four major types of addition reactions that can occur with alkenes, they include: Hydogenation, Halogenation, Hydrohalogenation, and Hydration.
In a Hydrogenation reaction, hydrogen H 2 is added across the double bond, converting an unsaturated molecule into a saturated molecule. Note that the word hydrogen is found in this reaction name, making it easier to remember and recognize: Hydrogen -ation. In a hydrogenation reaction, the final product is the saturated alkane.
In a Halogenation reaction group 7A elements the halogens are added across the double bond. The most common halogens that are incorporated include chlorine Cl 2 , bromine Br 2 , and Iodine I 2. Notice that the term halogen is found in this reaction name, making it easier to remember and recognize: Halogen -ation. In halogenation reactions the final product is haloalkane. In Hydrohalogenation , alkenes react with molecules that contain one hydrogen and one halogen.
Hence the name Hydro — Halogen -ation. HCl and HBr are common hydrohalogens seen in this reaction type. In hydrohalogenation, the hydrohalogen is a polar molecule, unlike the nonpolar molecules observed in the halogenation and hydrogenation reactions. In the case of the hydrohalogen, the end of the molecule containing hydrogen is partially positive, while the end of the molecule containing the halogen is partially negative.
Thus, when the negatively charged electron from the alkene double bond attacks the hydrohalogen, it will preferentially attack the hydrogen side of the molecule, since the electron will be attracted to the partial positive charge. The halogen will then form the negatively charged anion observed in the intermediate structure and attach second during the addition reaction.
The final product is a haloalkane. Just like when your are feeling thirsty, the terms hydration and dehydration refer to water. Hydration means the addition of water to a molecule, just like when you feel fully hydrated or full of water, while dehydration means the removal or elimination of water, just as when you are feeling dehydrated and need some water to drink.
Similar to the hydrohalogenation reaction above, water is also a polar molecule. In this case, the water is split into two groups to be added across the double bond of the alkene. It is split into the H- and the -OH components. Similar to the hydrohalogenation reaction, the hydrogen adds first, as it carries the partial positive charge.
In more complex molecules, hydrohalogenation and hydration reactions can lead the formation of more than one possible product. For example, if 2-methylpropene [ CH 3 2 CCH 2 ] reacts with water to form the alcohol, two possible products can form, as shown below.
However, the addition reaction is not random. One of the products is the major product being produced in higher abundance while the other product is the minor product. This occurs because the carbocation intermediate that forms as the reaction proceeds is more stable when it is bonded to other carbon atoms, than when it is bonded with hydrogen atoms, as seen in the example below:. In each reaction, the reagent adds across the double bond.
Write the equation for each reaction. What is the principal difference in properties between alkenes and alkanes? How are they alike? If C 12 H 24 reacts with HBr in an addition reaction, what is the molecular formula of the product? Alkenes undergo addition reactions; alkanes do not. Both burn. Complete each equation. In an elimination reaction a molecule loses a functional group, typically a halogen or an alcohol group, and a hydrogen atom from two adjacent carbon atoms to create an alkene structure.
Elimination reactions are essentially the reverse reaction of the hydration and hydrohalogenation addition reactions. Elimination reactions can also occur with the removal of water from alcohol.
A rearrangement reaction is a specific organic reaction that causes the alteration of the structure to form an isomer. With alkene structures, rearrangement reactions often result in the conversion of a cis -isomer into the trans conformation. Due to the high reactivity of alkenes, they usually undergo addition reactions rather than substitutions reactions.
The exception is the benzene ring. The double-bonded structure of the benzene ring gives this molecule a resonance structure such that all of the carbon atoms in the ring share a continually rotating partial bond structure. Thus, the overall structure is very stable compared to other alkenes and benzene rings do not readily undergo addition reactions. They behave more similarly to alkane structure and lack chemical reactivity.
One of the few types of reactions that a benzene ring will undergo is a substitution reaction. Recall from Chapter 7 that in substitution reactions an atom or group of atoms is replaced by another atom or group of atoms. Halogenation is a common substitution reaction that occurs with benzene ring structures.
In the diagram below, notice that the hydgrogen atom is substituted by one of the bromine atoms. Toluene gives Equivalent rate and product studies for other substitution reactions lead to similar conclusions. The manner in which specific substituents influence the orientation of electrophilic substitution of a benzene ring is shown in the following interactive diagram.
As noted on the opening illustration, the product-determining step in the substitution mechanism is the first step, which is also the slow or rate determining step.
It is not surprising, therefore, that there is a rough correlation between the rate-enhancing effect of a substituent and its site directing influence. The exact influence of a given substituent is best seen by looking at its interactions with the delocalized positive charge on the benzenonium intermediates generated by bonding to the electrophile at each of the three substitution sites. This can be done for seven representative substituents by using the selection buttons underneath the diagram.
In the case of alkyl substituents, charge stabilization is greatest when the alkyl group is bonded to one of the positively charged carbons of the benzenonium intermediate. This happens only for ortho and para electrophilic attack, so such substituents favor formation of those products. Interestingly, primary alkyl substituents, especially methyl, provide greater stabilization of an adjacent charge than do more substituted groups note the greater reactivity of toluene compared with tert-butylbenzene.
Structures in which like-charges are close to each other are destabilized by charge repulsion, so these substituents inhibit ortho and para substitution more than meta substitution.
Consequently, meta-products predominate when electrophilic substitution is forced to occur. Halogen X , OR and NR 2 substituents all exert a destabilizing inductive effect on an adjacent positive charge, due to the high electronegativity of the substituent atoms. By itself, this would favor meta-substitution; however, these substituent atoms all have non-bonding valence electron pairs which serve to stabilize an adjacent positive charge by pi-bonding, with resulting delocalization of charge.
Consequently, all these substituents direct substitution to ortho and para sites. The conditions commonly used for the aromatic substitution reactions discussed here are repeated in the table on the right. The electrophilic reactivity of these different reagents varies. Also, as noted earlier, toluene undergoes nitration about 25 times faster than benzene, but chlorination of toluene is over times faster than that of benzene.
From this we may conclude that the nitration reagent is more reactive and less selective than the halogenation reagents. Both sulfonation and nitration yield water as a by-product. This does not significantly affect the nitration reaction note the presence of sulfuric acid as a dehydrating agent , but sulfonation is reversible and is driven to completion by addition of sulfur trioxide, which converts the water to sulfuric acid.
The reversibility of the sulfonation reaction is occasionally useful for removing this functional group. The Friedel-Crafts acylation reagent is normally composed of an acyl halide or anhydride mixed with a Lewis acid catalyst such as AlCl 3. Such electrophiles are not exceptionally reactive, so the acylation reaction is generally restricted to aromatic systems that are at least as reactive as chlorobenzene.
Carbon disulfide is often used as a solvent, since it is unreactive and is easily removed from the product. If the substrate is a very reactive benzene derivative, such as anisole, carboxylic esters or acids may be the source of the acylating electrophile. Some examples of Friedel-Crafts acylation reactions are shown in the following diagram.
The first demonstrates that unusual acylating agents may be used as reactants. The second makes use of an anhydride acylating reagent, and the third illustrates the ease with which anisole reacts, as noted earlier.
The H 4 P 2 O 7 reagent used here is an anhydride of phosphoric acid called pyrophosphoric acid.
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