Dehydration Of 3 3 Dimethyl 2 Butanol
Lab Protocol #8 - Dehydration of 3,3-dimethyl-2-butanol
KEYWORDS: alkenes, E2, E1, carbocation stability, elimination
A. Introduction.
The dehydration of alcohols is an ELIMINATION reaction that is commonly used to form alkene molecules. The mechanism for the dehydration of alcohols consists of transforming the –OH functional group into a better leaving group by using a strong acid to protonate the alcohol. After –OH has been transformed into –OH2+, it becomes easier to break the σ-bond between the O and the C and for elimination to occur to form the alkene π-bond.
This elimination mechanism can proceed either via an E1 mechanism (two steps) or an E2 mechanism (one step) pathway (Scheme 1).
E1 (unimolecular)
H
+ H
H O
R C C R
R R
H
H O
R C C R
R R
R
H A
R
C C
R
+
+
B
R
H
+
R C C R
R R
E2 (bimolecular)
H
+ H
R O
R C C R
H R
H
R O
R C C R
H R
+
H A
+
B
R
R
C C
R
R
Scheme 1 – E1 and E2 Mechanistic Pathways.
The E1 pathway is a two-step mechanism that consists of a SLOW step (the leaving group (H2O) leaves to produce a carbocation) and a FAST step (a
Brønsted-Lowry base removes the β-proton and the new π-bond forms). This process is favored by carbocation stability and weaker Brønsted-Lowry bases.
In contrast, the E2 mechanism is a one-step simultaneous process: as the leaving group (H2O+) leaves, a Brønsted-Lowry base removes the β-proton and
1
Lab Protocol #8 - Dehydration of 3,3-dimethyl-2-butanol the new π-bond forms. This process is favored by stronger Brønsted-Lowry bases. The kinetics of the E1 mechanism are dependent upon carbocation formation, the slow step. Once they have been formed, unstable carbocations can rearrange to form more stable, more substituted carbocations. These carbocations will then result in more stable alkene formation and thermodynamically favor elimination (Zaitsev’s Rule). Carbocation rearrangement can occur via either a 1,2-methyl/alkyl shift or a 1,2-hydride shift (Scheme 2).
Unexpected product formation from
KEYWORDS: alkenes, E2, E1, carbocation stability, elimination
A. Introduction.
The dehydration of alcohols is an ELIMINATION reaction that is commonly used to form alkene molecules. The mechanism for the dehydration of alcohols consists of transforming the –OH functional group into a better leaving group by using a strong acid to protonate the alcohol. After –OH has been transformed into –OH2+, it becomes easier to break the σ-bond between the O and the C and for elimination to occur to form the alkene π-bond.
This elimination mechanism can proceed either via an E1 mechanism (two steps) or an E2 mechanism (one step) pathway (Scheme 1).
E1 (unimolecular)
H
+ H
H O
R C C R
R R
H
H O
R C C R
R R
R
H A
R
C C
R
+
+
B
R
H
+
R C C R
R R
E2 (bimolecular)
H
+ H
R O
R C C R
H R
H
R O
R C C R
H R
+
H A
+
B
R
R
C C
R
R
Scheme 1 – E1 and E2 Mechanistic Pathways.
The E1 pathway is a two-step mechanism that consists of a SLOW step (the leaving group (H2O) leaves to produce a carbocation) and a FAST step (a
Brønsted-Lowry base removes the β-proton and the new π-bond forms). This process is favored by carbocation stability and weaker Brønsted-Lowry bases.
In contrast, the E2 mechanism is a one-step simultaneous process: as the leaving group (H2O+) leaves, a Brønsted-Lowry base removes the β-proton and
1
Lab Protocol #8 - Dehydration of 3,3-dimethyl-2-butanol the new π-bond forms. This process is favored by stronger Brønsted-Lowry bases. The kinetics of the E1 mechanism are dependent upon carbocation formation, the slow step. Once they have been formed, unstable carbocations can rearrange to form more stable, more substituted carbocations. These carbocations will then result in more stable alkene formation and thermodynamically favor elimination (Zaitsev’s Rule). Carbocation rearrangement can occur via either a 1,2-methyl/alkyl shift or a 1,2-hydride shift (Scheme 2).
Unexpected product formation from