Question

On heating with alcoholic potassium hydroxide, neopentyl bromide gives mainly (1) 2 -methyl-2-butene (2) 2-mcthyl-1-butcne (3) 2 -butene (4) 2,2 -dimethyl-1-butene

Short answer

2,2-dimethyl-1-butene

Step-by-step solution

Understanding the Reaction

Neopentyl bromide is a primary alkyl halide. When heated with alcoholic potassium hydroxide (KOH), an elimination reaction, specifically an E2 mechanism, typically occurs.

Identify the Structural Formula of Neopentyl Bromide

Neopentyl bromide has the structure: CH3-C(CH3)2-CH2-Br It contains a bromine atom attached to a carbon atom which is bonded to two methyl groups and a methylene (CH2) group.

Predict the Elimination Product

In an E2 reaction, a base abstracts a proton from a β-carbon, and the leaving group (Br) is expelled to form a double bond. However, the neopentyl structure creates significant steric hindrance, which makes the elimination to form a double bond difficult.

Determine the Major Product

Given the steric hindrance around the bromine atom, elimination to form the more substituted alkene is unlikely. Thus, the less sterically hindered product formed is 2,2-dimethyl-1-butene (CH3-C(CH3)2-CH=CH2).

Key concepts

alcoholic potassium hydroxide

Alcoholic potassium hydroxide (KOH) is an essential reagent in organic chemistry, used primarily in elimination reactions like the E2 mechanism. In an E2 (bimolecular elimination) reaction, KOH acts as a strong base which abstracts a proton from the adjacent (beta) carbon to the carbon bonded with the leaving group (often a halide). The 'alcoholic' part refers to the solvent being alcohol, such as ethanol. This choice of solvent enhances the base strength of KOH because it prevents the ion from fully dissociating, thereby making it more nucleophilic. Using alcoholic KOH rather than aqueous KOH is crucial when the goal is elimination rather than substitution.

steric hindrance

Steric hindrance refers to the prevention of chemical reactions due to the spatial arrangement of atoms. In the context of the E2 elimination reaction, steric hindrance plays a significant role in determining the outcome. For neopentyl bromide, steric hindrance occurs because the carbon bonded to the bromine atom is surrounded by bulky methyl groups, making it difficult for the base to access and abstract a proton. This difficulty results in a preference for forming less substituted, less sterically hindered alkenes as the reaction products.

primary alkyl halide

A primary alkyl halide is an organic compound wherein the carbon atom attached to the halogen (such as bromine) is connected to only one other carbon atom. This makes primary alkyl halides generally more reactive in substitution reactions due to less steric hindrance and weaker hyperconjugation. However, in elimination reactions like E2, their reactivity and the product distribution can be strongly influenced by adjacent groups and steric effects. Neopentyl bromide (CH3-C(CH3)2-CH2-Br) is an example of a primary alkyl halide, where the presence of bulky groups adjacent to the halogen-bearing carbon affects its behavior under elimination conditions.

neopentyl bromide

Neopentyl bromide (CH3-C(CH3)2-CH2-Br) is a primary alkyl halide characterized by a central carbon atom bonded to three other carbon atoms and one bromine atom. The bulky groups (two methyl and one methylene) create significant steric hindrance, making certain chemical reactions challenging. In the context of an E2 elimination reaction with alcoholic KOH, this steric hindrance prevents the formation of the more substituted alkene. Instead, it yields the less hindered product, 2,2-dimethyl-1-butene. Understanding the structure and its implications helps predict the reaction pathways accurately.

2,2-dimethyl-1-butene

2,2-Dimethyl-1-butene is an organic compound resulting from the E2 elimination reaction of neopentyl bromide. Its structure can be represented as CH3-C(CH3)2-CH=CH2. This particular alkene forms because of the significant steric hindrance around the neopentyl bromide, which prevents the formation of a more highly substituted double bond. In an E2 reaction, the base (like alcoholic KOH) abstracts a proton from a beta-carbon, leading to the expulsion of the bromine atom and formation of this less sterically hindered double bond. Knowing the structure and steric factors allows better understanding and prediction of the reaction outcomes.