Question

The expected increasing order of hydration of ethene, propene and 2 -methyl propene is as (1) propene \(<\) cthenc \(<2\) -mcthyl propenc (2) cthene < propene \(<2\) -mcthyl propenc (3) 2 -methyl propenc \(<\) cthene \(<\) propene (4) 2 -methyl propenc \(<\) propene \(<\) cthene

Short answer

The correct order is ethene < propene < 2-methyl propene (Option 2).

Step-by-step solution

Understand Hydration

Hydration refers to the addition of water to a compound. In alkenes, this usually means the addition of water across the double bond, leading to the formation of an alcohol.

Stability of Carbocations

When an alkene undergoes hydration, a carbocation intermediate is formed. The stability of the carbocation intermediate affects the rate of hydration. Tertiary carbocations are more stable than secondary, which are more stable than primary.

Analyze Each Alkene

Ethene forms a primary carbocation, propene forms a secondary carbocation, and 2-methylpropene forms a tertiary carbocation. Hence, the stability order is: tertiary > secondary > primary.

Determine Increasing Order of Hydration

Since the hydration rate is directly related to carbocation stability, the increasing order of hydration will be from primary to tertiary carbocations. Thus, ethene (primary) < propene (secondary) < 2-methyl propene (tertiary).

Key concepts

hydration

Hydration is a crucial concept in organic chemistry, especially in reactions involving alkenes. It refers to the addition of water (H₂O) to a compound. For alkenes, hydration usually results in the formation of an alcohol. This process involves breaking the double bond in the alkene and adding an OH group to one of the carbons and an H to the other carbon.
A common example is the hydration of ethene to form ethanol. The water molecule splits into H⁺ and OH⁻ ions, which then add to the carbon atoms of the double bond. This is a simple yet vital reaction mechanism that reveals how functional groups can be introduced into hydrocarbon chains.

alkenes

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond (C=C). This double bond is what gives alkenes their reactivity and versatility in chemical reactions.
The general formula for alkenes is CnH2n. For example, ethene (C₂H₄), propene (C₃H₆), and 2-methylpropene (C₄H₈). Since they are unsaturated compounds, alkenes can participate in addition reactions where atoms or groups of atoms are added to the carbons of the double bond.
In many cases, the position of the double bond and the structure of the surrounding carbon chain will affect the reaction outcome, such as in the hydration process.

carbocation stability

Carbocations are intermediates in many organic reactions, including the hydration of alkenes. A carbocation is a positively charged carbon atom, formed when a carbon-carbon double bond breaks and one of the carbons gets an extra positive charge.
The stability of carbocations plays a key role in determining reaction rates. Stability increases with the number of alkyl groups attached to the positively charged carbon. Thus, the order of stability is:

• Tertiary carbocation (3°): Carbon surrounded by three alkyl groups
• Secondary carbocation (2°): Carbon surrounded by two alkyl groups
• Primary carbocation (1°): Carbon surrounded by one alkyl group

Given this, a tertiary carbocation from 2-methylpropene is more stable than a secondary carbocation from propene, which in turn is more stable than a primary carbocation from ethene.

reaction mechanism

The reaction mechanisms for alkene hydration usually follow a series of well-defined steps. First, the alkene undergoes protonation where an H⁺ ion from an acid catalyst (often H₂SO₄) adds to one of the carbons in the double bond. This leads to the formation of a carbocation intermediate.
The next step involves nucleophilic attack by a water molecule on the carbocation, resulting in the formation of a protonated alcohol. Finally, deprotonation occurs to yield the final alcohol product and regenerate the acid catalyst.
Understanding these steps is essential for grasping how different alkenes will react. For instance, ethene will form a primary carbocation, propene will form a secondary carbocation, and 2-methylpropene will form a tertiary carbocation when hydrated. The more stable the carbocation, the faster the reaction proceeds, therefore impacting the order of hydration: ethene < propene < 2-methylpropene.