Q.Assertion: The C-O-C bond angle in ethers is slightly less than the tetrahedral angle.
Reason: There is a repulsion between the two bulky (-R) groups.

Concept: Boiling Point Trends — here, the focus is on bond angles in ethers, not boiling points. The key idea is steric repulsion between alkyl groups.

Reasoning:

1. The tetrahedral angle is $109.5^\circ$. In ethers ($R-O-R$), the C-O-C bond angle is about $110^\circ$ to $112^\circ$ — actually slightly larger than the tetrahedral angle, not smaller.
2. This increase arises because the two bulky alkyl groups repel each other, pushing the bond angle open. …

Why this question trips students up

Most students memorise that ethers have a bent shape around oxygen, and they vaguely recall "tetrahedral angle = 109.5°". The trap is assuming that because oxygen has two lone pairs (like in water), the bond angle must be smaller than tetrahedral — just like water's angle is 104.5°. But ethers are not water. The key difference: in ethers, the two groups attached to oxygen are alkyl groups (R), not hydrogens. Those alkyl groups are bulky and repel each other, pushing the C-O-C angle open.

Let's break it down properly.

1. Recall the reference: the tetrahedral angle

   In a perfect tetrahedron (e.g., methane, CH₄), the bond angle is $109.5^\circ$. This is the angle when four identical groups are equally spaced around a central atom.

2. What is the actual C-O-C bond angle in ethers?

   In dimethyl ether (CH₃–O–CH₃), the C-O-C bond angle is about $111^\circ$–$112^\circ$. In diethyl ether, it's around $110^\circ$. These are slightly greater than $109.5^\circ$, not less.

3. Why is it greater, not smaller?

   Oxygen in ethers is $sp^3$ hybridised — it has two bond pairs (to the two R groups) and two lone pairs. Lone pairs repel more strongly than bond pairs (VSEPR theory). That repulsion would normally compress the bond angle (as in water, where lone pair–lone pair repulsion gives $104.5^\circ$).

   But here, the two R groups are large and bulky. Their steric repulsion overcomes the lone pair compression, forcing the C-O-C angle to open up beyond $109.5^\circ$.

4. Evaluate the Assertion …

Method: Conceptual Evaluation of Assertion-Reason Statements

This method is used to test both the factual correctness of two statements and the logical link between them.

Steps:

1. Verify the Assertion

   • The C–O–C bond angle in ethers (like dimethyl ether) is about 110°–112° — slightly more than the tetrahedral angle of 109.5°, because the two bulky alkyl groups repel each other and widen the angle.
   • Conclusion: The assertion is wrong.

2. Verify the Reason

   • The reason states that repulsion between two bulky (–R) groups increases the bond angle.
   • This is chemically correct — larger groups push each other apart, widening the angle.
   • Conclusion: The reason is correct.

3. Check the Logical Link …

Here’s a breakdown of the common mistakes students make on this question and how to avoid each.

Mistake 1: Thinking the C-O-C bond angle is exactly tetrahedral (109.5°)

Why students get it wrong:

They assume that because oxygen is $sp^3$ hybridized in ethers, the bond angle must be exactly $109.5^\circ$.

How to avoid:

Remember that lone pairs repel more strongly than bond pairs. In ethers, oxygen has two lone pairs and two bond pairs. The lone pair–bond pair repulsion compresses the C-O-C angle to about 110°–112° (slightly more than tetrahedral, not less).

• The assertion says “slightly less than tetrahedral” — this is incorrect.
• The correct fact: The angle is slightly more than $109.5^\circ$ due to lone pair repulsion.

Mistake 2: Confusing “bulky group repulsion” with lone pair repulsion

Why students get it wrong:

They see “two bulky (-R) groups” and think “repulsion → angle decreases” — which sounds logical, but it’s not the dominant effect here.

How to avoid:

• In ethers, the lone pair repulsion is stronger than the steric repulsion between alkyl groups.
• Bulky groups would increase the angle if they were the main factor, but they are not.
• The actual C-O-C angle is ~110°–112° (greater than tetrahedral), so the reason given (repulsion between bulky groups) is not the correct explanation — even if it sounds plausible.

Mistake 3: Assuming the reason is completely false

Why students get it wrong:

They know the assertion is wrong, so they assume the reason must also be wrong.

How to avoid:

• The reason statement: “There is a repulsion between the two bulky (-R) groups” is factually correct — such repulsion does exist.
• But it is not the reason for the bond angle being less than tetrahedral (because the angle is actually greater).
• So the reason is a correct statement, but it does not explain the (incorrect) assertion.

Mistake 4: Picking option (A) or (B) without checking the assertion’s truth

Why students get it wrong:

They focus only on the reason and forget to verify the assertion independently.

How to avoid: …