Organic Chem

Topics: Alcohol, Nucleophilic substitution, SN1 reaction Pages: 13 (2610 words) Published: June 2, 2013
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

+ H2O

oxonium ion

planar carbocation

The third step is the attack of the planar carbocation by X- (in our case the halide ion).

X

R3 R4

R1 R2 H

X R3 R4

R1 R2 H

 

1-1
 

Chemistry 2283g

Experiment 1 – Alkyl Halides

This mechanism is followed when R is a tertiary alkyl group and is called SN1 (substitution, nucleophilic, unimolecular in the rate-determining step). An SN1 mechanism may also be involved when R is a secondary group or when R can form a resonance-stabilized carbocation, such as an allyl or benzyl cation (the more stable the carbocation, the more likely the SN1 mechanism). If R is a primary alkyl group, then substitution occurs generally without formation of the carbocation by an SN2 mechanism (why no carbocation? Because o 1 carbocations are generally unstable). In this case a halide ion attacks the primary carbon atom of the oxonium ion in the rate-determining step, and the alkyl halide forms directly (why is this mechanism not favorable when R = tertiary?). Because two species, the nucleophile (X-) and the substrate (the protonated alcohol), are involved in the rate-determining step, the mechanism is designated SN2. When R is a secondary group, an SN1 or an SN2 mechanism may be involved, depending on the reaction conditions. SN2 Mechanism:

X

H

HO H H

R1 R2 H

H2O H H X

R1 R2 H

H H X

R1 R2 H

+ H2O

Experiment: Preparation and Reactivity of Alkyl Halides In this experiment you will prepare 1-bromobutane (1-butyl bromide) from 1-butanol shown in equation 2 (by what mechanism do you expect this substitution reaction to occur?). Compound 1-butanol is a primary alcohol and thus one might expect that the reaction would proceed by an SN2 reaction. However, when a strong, non-nucleophilic, acid is used as a solvent the conditions are very polar and the reaction can proceed via an SN1 reaction. You will use the NMR spectrum of your product and gas chromatography results to make decisions...
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