Question 27.1: Identifying Types of Organic Reactions. State whether each o......
Identifying Types of Organic Reactions
State whether each of the following reactions is a substitution, an addition, an elimination, or a rearrangement reaction. (In the equations below, H^{+} and Pd are catalysts.)
(a) (CH_{3})_{2}CH\overset{\begin{matrix} O \\ \parallel \end{matrix} }{C}ONA + CH_{3}Br \longrightarrow (CH_{3})_{2}CH\overset{\begin{matrix} O \\ \parallel \end{matrix} }{C}OCH_{3} + NaBr
(b) 
(c) 2 CH_{3}OH \xrightarrow[\Delta]{H^{+}} CH_{3}OCH_{3} + H_{2}O
(d) cyclohexanol \xrightarrow[\Delta]{H^{+}} cyclohexene + water
(e) but-2-yne + 2 H_{2} \xrightarrow[]{Pd} butane
Analyze
In a substitution reaction, an atom or a group of atoms is replaced with another atom or group of atoms. In an addition reaction, atoms add across a double or triple bond, causing a reduction in bond order between two atoms. In an elimination reaction, atoms or groups of atoms that are bonded to adjacent atoms are eliminated, causing an increase in bond order between two atoms. Typically, a rearrangement reaction involves a change in constitution or stereochemical configuration.
Solve
(a) Because a Br atom in CH_{3}Br is replaced by a (CH_{3})_{2}CHCOO group, the reaction is a substitution reaction.
(b) The reaction involves only a change in the skeletal structure (constitution), and so the reaction is a rearrangement reaction.
(c) When we write the reaction out as CH_{3}OH + CH_{3}OH → CH_{3}OCH_{3} + H_{2}O, we see that an —OH group in one molecule is replaced by a —CH_{3}O group. The reaction is a substitution reaction.
(d) Here, we need to convert the names of the compounds into structural formulas.
The reaction involves elimination of OH and H from adjacent carbon atoms in cyclohexanol. Thus, the reaction is an elimination reaction.
(e) In this reaction, an alkyne is converted into an alkane. Because there is a reduction in bond order, we know that the reaction is an addition reaction involving the addition of H atoms across the triple bond.
Our prediction is confirmed by writing the reaction in terms of structural formulas:
CH_{3}C≡CCH_{3} + 2 H_{2} → CH_{3}CH_{2}CH_{2}CH_{3}
Assess
To identify the reaction type correctly, it is often helpful to write out structural formulas for all molecules involved.