ALCOHOLS AND ALKYL HALIDES

1 There are four C4H9 alkyl groups, and so there are four C4H9C1 alkyl chlorides. Each may be named 

CH3CH2CH2CH2C1 n-Butyl chloride (Butyl chloride) 1-Chlorobutane 

Cl tert-Butyl chloride (1,1-Dimethylethyl chloride) 2-Chloro-2-methylpropane 

2 Alcohols may also be named using both the functional class and substitutive methods, as in the previous problem. 

3 Alcohols are classified as primary, secondary, or tertiary according to the number of carbon substituents attached to the carbon that bears the hydroxyl group. 

Two reactions that lead to alkyl halides will be described in this chapter. Both illustrate functional group transformations, fii the first, the hydroxyl group of an alcohol is replaced by halogen on treatment with a hydrogen halide. 

In the second, reaction with chlorine or bromine causes one of the hydrogen substituents of an alkane to be replaced by halogen. 

Both reactions are classified as substitutions, a term that describes the relationship between reactants and products—one functional group replaces another. In this chapter we go beyond the relationship of reactants and products and consider the mechanism of each reaction. A mechanism attempts to show how starting materials are converted into products during a chemical reaction. 

While developing these themes of reaction and mechanism, we will also use alcohols and alkyl halides as vehicles to extend the principles of lUPAC nomenclature, con- 

The lUPAC mles permit alkyl halides to be named in two different ways, called functional class nomenclature and substitutive nomenclature. In functional class nomenclature the aUcyl group and the halide (fluoride, chloride, bromide, or iodide) are designated as separate words. The alkyl group is named on the basis of its longest continuous chain beginning at the carbon to which the halogen is attached. 

6 Physical Properties of Alcohols and Alkyl Halides Intermolecular Forces 143 

7 Preparation of Alkyl Halides from Alcohols and Hydrogen Halides 147 

8 Mechanism of the Reaction of Alcohols with Hydrogen Halides  

9 Potential Energy Diagrams for Multistep Reactions The SnI Mechanism 153 

---

Organic compounds are grouped into families according to the functional groups they contain Two of the most important families are alcohols and alkyl halides Alco hols and alkyl halides are especially useful because they are versatile starting materials for preparing numerous other families Indeed alcohols or alkyl halides—often both— will appear m virtually all of the remaining chapters of this text... 

It IS convenient m equations such as this to represent generic alcohols and alkyl halides as ROH and RX respectively where R stands for an alkyl group In addition to con venience this notation lets us focus more clearly on the functional group transformation that occurs the OH functional group of an alcohol is replaced as a substituent on car bon by a halogen usually chlorine (X = Cl) or bromine (X = Br)... 

While developing the connections between structure reaction and mechanism we will also extend the fundamentals of lUPAC nomenclature to functional group families beginning with alcohols and alkyl halides... 

Alcohols and alkyl halides are classified as primary secondary or tertiary according to the degree of substitution of the carbon that bears the functional group (Section 2 13) Thus primary alcohols and primary alkyl halides are compounds of the type RCH2G (where G is the functional group) secondary alcohols and secondary alkyl halides are compounds of the type R2CHG and tertiary alcohols and tertiary alkyl halides are com pounds of the type R3CG... 

Many of the properties of alcohols and alkyl halides are affected by whether then-functional groups are attached to primary secondary or tertiary carbons We will see a number of cases m which a functional group attached to a primary carbon is more reac live than one attached to a secondary or tertiary carbon as well as other cases m which the reverse is true... 

The carbon that bears the functional group is sp hybridized m alcohols and alkyl halides Figure 4 1 illustrates bonding m methanol The bond angles at carbon are approximately tetrahedral as is the C—O—H angle A similar orbital hybridization model applies to alkyl halides with the halogen connected to sp hybridized carbon by a ct bond Carbon-halogen bond distances m alkyl halides increase m the order C—F (140 pm) < C—Cl (179 pm) < C—Br (197 pm) < C—I (216 pm)... 

Carbon-oxygen and carbon-halogen bonds are polar covalent bonds and carbon bears a partial positive charge in alcohols ( " C—0 ) and in alkyl halides ( " C—X ) Alcohols and alkyl halides are polar molecules The dipole moments of methanol and chloromethane are very similar to each other and to water... 

Relatively simple notions of attractive forces between opposite charges are suffi cient to account for many of the properties of chemical substances You will find it help ful to keep the polarity of carbon-oxygen and carbon-halogen bonds m mind as we develop the properties of alcohols and alkyl halides m later sections... 

PHYSICAL PROPERTIES OF ALCOHOLS AND ALKYL HALIDES INTERMOLECULAR FORCES... 

---