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Chemical reactivity and functional group transformations involving the preparation of alkyl halides from alcohols and from alkanes are the main themes of this chapter. Although the conversions of an alcohol or an alkane to an alkyl halide are both classified as substitutions, they proceed by very different mechanisms.

Section 4.1

Functional groups are the structural units responsible for the characteristic reactions of a molecule. The hydrocarbon chain to which a functional group is attached can often be considered as simply a supporting framework. The most common functional groups characterize the families of organic compounds listed on the inside front cover of the text.

Section 4.2

Alcohols and alkyl halides may be named using either substitutive or functional class IUPAC nomenclature. In substitutive nomenclature alkyl halides are named as halogen derivatives of alkanes. The parent is the longest continuous chain that bears the halogen substituent, and in the absence of other substituents the chain is numbered from the direction that gives the lower number to the carbon that bears the halogen. The functional class names of alkyl halides begin with the name of the alkyl group and end with the halide as a separate word.

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Section 4.3

The substitutive names of alcohols are derived by replacing the -e ending of an alkane with -ol. The longest chain containing the OH group becomes the basis for the name. Functional class names of alcohols begin with the name of the alkyl group and end in the word alcohol.

Section 4.4

Alcohols (X = OH) and alkyl halides (X = F, Cl, Br, or I) are classified as primary, secondary, or tertiary according to the degree of substitution at the carbon that bears the functional group.

Section 4.5

The halogens (especially fluorine and chlorine) and oxygen are more electronegative than carbon, and the carbon–halogen bond in alkyl halides and the carbon–oxygen bond in alcohols are polar. Carbon is the positive end of the dipole and halogen or oxygen the negative end.

Section 4.6

Dipole/induced-dipole and dipole–dipole attractive forces make alcohols higher boiling than alkanes of similar molecular size. The attractive force between —OH groups is called hydrogen bonding.

Hydrogen bonding between the hydroxyl group of an alcohol and water makes the water-solubility of alcohols greater than that of hydrocarbons. Low-molecular-weight alcohols [CH3OH, CH3CH2OH, CH3CH2CH2OH, and (CH3)2CHOH] are soluble in water in all proportions. Alkyl halides are insoluble in water.

Section 4.7

See Table 4.4

Conversions of Alcohols and Alkanes to Alkyl Halides
Reaction (section) and comments General equation and specific example(s)
Reactions of alcohols with hydrogen halides (Section 4.7) Alcohols react with hydrogen halides to yield alkyl halides. The reaction is useful as a synthesis of alkyl halides. The reactivity of hydrogen halides decreases in the order Hl > HBr > HCl > HF. Alcohol reactivity decreases in the order tertiary > secondary > primary.
Reaction of alcohols with thionyl chloride (Section 4.14) Thionyl chloride converts alcohols to alkyl chlorides.
Reaction of alcohols with phosphorus tribromide (Section 4.14) As an alternative to converting alcohols to alkyl bromides with hydrogen bromide, the inorganic reagent phosphorus tribromide is sometimes used.
Free-radical halogenation of alkanes (Sections 4.15 through 4.19) Alkanes react with halogens by substitution of a halogen for a hydrogen on the alkane. The reactivity of the halogens decreases in the order F2 > Cl2 > Br2 > l2. The ease of replacing a hydrogen decreases in the order tertiary > secondary > primary > methyl. Chlorination is not very selective and so is used only when all the hydrogens of the alkane are equivalent. Bromination is highly selective, replacing tertiary hydrogens much more readily than secondary or primary ones.
Section 4.8

Secondary and tertiary alcohols react with hydrogen halides by a mechanism that involves formation of a carbocation intermediate in the rate-determining step.

Section 4.9

The potential energy diagrams for separate elementary steps can be merged into a diagram for the overall process. The diagram for the reaction of a secondary or tertiary alcohol with a hydrogen halide is characterized by two intermediates and three transition states. The reaction is classified as a unimolecular nucleophilic substitution, abbreviated as SN1.

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Section 4.10

Carbocations contain a positively charged carbon with only three atoms or groups attached to it. This carbon is sp2-hybridized and has a vacant 2p orbital.

Carbocations are stabilized by alkyl substituents attached directly to the positively charged carbon. Alkyl groups are electron-releasing substituents. Stability increases in the order:

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Carbocations are strong electrophiles (Lewis acids) and react with nucleophiles (Lewis bases).

Section 4.11

The rate at which alcohols are converted to alkyl halides depends on the rate of carbocation formation: tertiary alcohols are most reactive; primary alcohols are least reactive.

Section 4.12

Primary alcohols and methanol do not react with hydrogen halides by way of carbocation intermediates. The nucleophilic species (Br for example) attacks the alkyloxonium ion and displaces a water molecule from carbon in a bimolecular step. This step is rate-determining, and the mechanism is SN2.

Section 4.13

A quantitative relationship between the energy of activation (Eact), the rate constant (k) and temperature (T) is expressed by the Arrhenius equation:

An increase in temperature will increase the value of the rate constant (k). Small differences in activation energy lead to large differences in reaction rate.

Section 4.14

See Table 4.4

Section 4.15

See Table 4.4

Section 4.16

Methane reacts with Cl2 to give chloromethane, dichloromethane, trichloromethane, and tetrachloromethane.

Section 4.17

Chlorination of methane, and halogenation of alkanes generally, proceed by way of free-radical intermediates. Alkyl radicals are neutral and have an unpaired electron on carbon.

Like carbocations, free radicals are stabilized by alkyl substituents. The order of free-radical stability parallels that of carbocation stability.

Section 4.18

The elementary steps (1) through (3) describe a free-radical chain mechanism for the reaction of an alkane with a halogen.

Section 4.19

See Table 4.4

Descriptive Passage and Interpretive Problems 4
More About Potential Energy Diagrams

Chapter 5 describes elimination reactions and their mechanisms. In one example, heating tert-butyl bromide in ethanol gives the alkene 2-methylpropene by a two-step mechanism:

Step 1:
Step 2:

A potential energy diagram for the reaction provides additional information to complement the mechanism expressed in the equations for the two elementary steps.

The energy relationships in the diagram are not only useful in their own right, but also aid in understanding the structural changes occurring at the transition state. Hammond's postulate tells us that if two states occur consecutively, the closer they are in energy, the more similar they are in structure.

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  • 4.55

    Which equation corresponds to the overall reaction for which steps 1 and 2 describe the mechanism?

  • 4.56

    Ethanol is:

    1. a catalyst

    2. a reactive intermediate

    3. a Brønsted acid

    4. a Brønsted base

  • 4.57

    According to the potential energy diagram, the overall reaction is:

    1. endothermic

    2. exothermic

  • 4.58

    Classify the elementary steps in the mechanism according to their molecularity.

    1. Step 1 is unimolecular; step 2 is bimolecular.

    2. Step 1 is bimolecular; step 2 is unimolecular.

    3. Both steps are unimolecular.

    4. Both steps are bimolecular.

  • 4.59

    Classify states 2–4 in the potential energy diagram.

    1. 2, 3, and 4 are transition states

    2. 2, 3, and 4 are reactive intermediates

    3. 2 and 4 are transition states; 3 is a reactive intermediate

    4. 2 and 4 are reactive intermediates; 3 is a transition state

  • 4.60

    According to the diagram, the activation energy of the slow step is given by the energy difference between states

    1. 1 and 2

    2. 2 and 3

    3. 3 and 4

    4. 1 and 5

  • 4.61

    What best describes the species at the rate-determining transition state?

  • 4.62

    By applying Hammond's postulate to the potential energy diagram for this reaction, we can say that:

    1. the structure of 2 is more carbocation-like than 4

    2. the structure of 2 is less carbocation-like than 4

    3. the structure of 2 resembles 1 more than it resembles 3

    4. the structure of 4 resembles 5 more than it resembles 3