Organic Chem
Chemistry 2283g
Experiment 1 – Alkyl Halides
EXPERIMENT 1: Preparation and Reactivity of Alkyl Halides
Relevant Sections in the text (Wade, 7 ed.) • 6.7 (p. 228) Reactions of alkyl halides • 6.8 – 6.12 (p. 229 – 242) The SN2 reaction: generality, factors affecting SN2 reactions, substrate reactivity, mechanism • 6.13 – 6.14 (p. 243 – 249) The SN1 reaction: mechanism, stereochemistry General Concepts The most common of the variety of methods available for preparing alkyl halides is the replacement of the hydroxyl group of an alcohol. This type of reaction is representative of an important class of reactions in organic chemistry called nucleophilic aliphatic substitution reactions. There are numerous ways of doing this transformation, and we will discuss these in lecture. In this experiment you will convert an alcohol to an alkyl halide by reaction with a hydrohalic acid. The overall reaction is shown in equation 1; however the mechanism of the reaction depends on the structure of the alkyl group bearing the functional group being replaced. R−OH + H-X → R−X + H2O (1) th The reaction may occur by one of two mechanisms designated SN1 or SN2. Which mechanism operates depends on the structure of the R group and the reaction conditions. The first step in both mechanisms is the protonation of the alcohol to form an oxonium ion, converting the OH group into a good leaving group. What happens next depends on the nature of the alkyl group, R. If R is a group that readily forms a carbocation, then the slow, rate-determining step is the loss of a water molecule from the oxonium ion. Once formed, the carbocation then reacts rapidly with a halide ion to form the alkyl halide. SN1 Mechanism: The first step is protonation of the alcohol, followed by the second step which is the formation of the carbocation via the oxonium ion. This second step is the slow step (rate-determining).
X
H
HO R3 R4
R1 R2 H
H2O R3 R4
R1 R2 H
RDS + X
R3 R4
R1 R2 H
+
Experiment 1 – Alkyl Halides
EXPERIMENT 1: Preparation and Reactivity of Alkyl Halides
Relevant Sections in the text (Wade, 7 ed.) • 6.7 (p. 228) Reactions of alkyl halides • 6.8 – 6.12 (p. 229 – 242) The SN2 reaction: generality, factors affecting SN2 reactions, substrate reactivity, mechanism • 6.13 – 6.14 (p. 243 – 249) The SN1 reaction: mechanism, stereochemistry General Concepts The most common of the variety of methods available for preparing alkyl halides is the replacement of the hydroxyl group of an alcohol. This type of reaction is representative of an important class of reactions in organic chemistry called nucleophilic aliphatic substitution reactions. There are numerous ways of doing this transformation, and we will discuss these in lecture. In this experiment you will convert an alcohol to an alkyl halide by reaction with a hydrohalic acid. The overall reaction is shown in equation 1; however the mechanism of the reaction depends on the structure of the alkyl group bearing the functional group being replaced. R−OH + H-X → R−X + H2O (1) th The reaction may occur by one of two mechanisms designated SN1 or SN2. Which mechanism operates depends on the structure of the R group and the reaction conditions. The first step in both mechanisms is the protonation of the alcohol to form an oxonium ion, converting the OH group into a good leaving group. What happens next depends on the nature of the alkyl group, R. If R is a group that readily forms a carbocation, then the slow, rate-determining step is the loss of a water molecule from the oxonium ion. Once formed, the carbocation then reacts rapidly with a halide ion to form the alkyl halide. SN1 Mechanism: The first step is protonation of the alcohol, followed by the second step which is the formation of the carbocation via the oxonium ion. This second step is the slow step (rate-determining).
X
H
HO R3 R4
R1 R2 H
H2O R3 R4
R1 R2 H
RDS + X
R3 R4
R1 R2 H
+