Problem 121 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Calculate total number of \(\alpha\) -H present in alkene formed when 2, 3-dimethyl butanol react with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4} / \Delta\)

Step-by-Step Solution

Answer

The alkene formed is 2,3-dimethyl-2-butene with 6 \(\alpha\)-hydrogens.

Step 1: Understand the Reaction Type

The reaction is a dehydration of alcohol, where 2,3-dimethyl butanol in the presence of concentrated sulfuric acid (\(\mathrm{H}_{2} \mathrm{SO}_{4}/ \Delta \)) loses a water molecule to form an alkene.

Step 2: Identify the Structure of 2,3-dimethyl butanol

The structure of 2,3-dimethyl butanol is CH3-CH(CH3)-CH(CH3)-CH2OH. It is a 4-carbon alcohol with methyl groups on the 2nd and 3rd carbons.

Step 3: Determine the Alkene Product

Dehydration occurs primarily at the OH-bearing carbon and an adjacent carbon (due to Zaitsev's Rule which states the more substituted alkene is favored). The major product is 2,3-dimethyl-2-butene.

Step 4: Draw the Structure of the Alkene

The structure of 2,3-dimethyl-2-butene is \( CH_3-C(CH_3)=C(CH_3)-CH_3 \). This has a double bond between the second and third carbons, making it more substituted.

Step 5: Calculate the Number of \(\alpha\)-Hydrogens

\(\alpha\)-Hydrogens are the hydrogen atoms attached to the \(\alpha\)-carbon, which is the carbon adjacent to the double-bonded carbons. In 2,3-dimethyl-2-butene, the \(\alpha\)-carbons are the methyl groups attached to the second and third carbons. Each methyl (CH3) group contains 3 hydrogens, leading to a total of 6 \(\alpha\)-hydrogens.

Key Concepts

Dehydration Reaction

A dehydration reaction is an essential concept in organic chemistry where a water molecule is removed from a compound. This often involves alcohols turning into alkenes, and is catalyzed by acids like concentrated sulfuric acid. 2,3-dimethyl butanol undergoes dehydration when treated with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}/ \Delta\), meaning heat may be applied to facilitate the reaction. During this process:

• The hydroxyl group (OH) of the alcohol is protonated by the acid, increasing its leaving capability.


• A water molecule is eliminated, forming an unstable carbocation intermediate.


• The carbocation then loses a hydride ion (H⁻), forming a double bond hence an alkene.

Understanding the mechanism helps students predict the outcome and the structure of the resultant alkene.

Zaitsev's Rule

In organic chemistry, selecting which hydrogen atom to eliminate during a dehydration reaction is often determined by Zaitsev's Rule. This rule posits that the more substituted alkene, one with the most non-hydrogen substituent groups on the double-bonded carbons, is the favored product.
In the reaction involving 2,3-dimethyl butanol, the major product, 2,3-dimethyl-2-butene, exemplifies this rule.

• This alkene is more substituted due to the presence of methyl groups at the double-bonded carbons.


• By following Zaitsev's Rule, you can predict that the alkene formed will be more stable and thus predominantly produced.

This concept is crucial as it guides the synthesis of specific alkenes in laboratory settings, ensuring more efficient production of desired compounds.

Alpha Hydrogen Calculation

Alpha hydrogens are counted to understand the reactivity of a substance, particularly in understanding stability and kinetic aspects of a reaction. To calculate \( \alpha \)-hydrogens in the given alkene, 2,3-dimethyl-2-butene, they are located on the carbons adjacent to the double bond.
These adjacent carbons have been termed \( \alpha \) carbons, and the hydrogens attached are the \( \alpha \)-hydrogens. The 2,3-dimethyl-2-butene structure consists of methyl groups \( (CH_3) \) on each side of the double bond:

• Each methyl group is attached to the \( \alpha \) carbons and has 3 hydrogens.


• With two such groups, there is a total of 6 \( \alpha \)-hydrogens.

Calculating \( \alpha \)-hydrogens is crucial for understanding potential reactions and the formation of intermediates, such as in subsequent steps of organic reactions.