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NCERT Exemplar · Q46

Q.Why is the C-O-H bond angle in alcohols slightly less than the tetrahedral angle whereas the C-O-C bond angle in ether is slightly greater?

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The bond angle around oxygen is determined by the balance between lone‑pair repulsion and the steric/electronic nature of the attached groups. In alcohols, the small H atom allows lone‑pair repulsion to compress the C‑O‑H angle below 109.5°. In ethers, two bulky alkyl groups push the C‑O‑C angle open slightly beyond the tetrahedral value.

The core idea: oxygen’s lone pairs are the real boss

Oxygen in both alcohols and ethers is sp3sp^3 hybridised — it has two sigma bonds and two lone pairs. According to VSEPR theory, the four electron domains arrange themselves tetrahedrally (ideal angle 109.5∘109.5^\circ). But the actual bond angle depends on two competing forces:

  1. Lone‑pair repulsion – lone pairs occupy more space than bonding pairs, so they push the bonded atoms closer together.
  2. Steric repulsion – bulky groups attached to oxygen physically push each other apart, opening the angle.

The final angle is the net result of these two effects. The difference between alcohols and ethers comes down to what is attached to the oxygen.

Step‑by‑step reasoning

  1. Hybridisation and geometry of oxygen

    In both R–OH and R–O–R′, the oxygen is sp3sp^3 hybridised. The four sp3sp^3 orbitals point toward the corners of a tetrahedron. Two of these orbitals contain lone pairs; the other two form sigma bonds. If all four substituents were identical, the bond angle would be exactly 109.5∘109.5^\circ.

  2. Why the C–O–H angle in alcohols is less than 109.5°

    The hydrogen atom in the O–H group is very small — it offers almost no steric resistance. The lone pairs, which are closer to the oxygen nucleus and more diffuse than bonding pairs, exert a stronger repulsive force on each other and on the bonding pairs. This lone‑pair repulsion compresses the C–O–H angle.

    Watch out

    A common mistake is to think that the O–H bond itself is “weak” or “bent” — it isn’t. The compression comes purely from the lone pairs dominating the space around oxygen.

    The observed angle in methanol, for example, is about 108.9∘108.9^\circ, and in most alcohols it lies in the range 108∘108^\circ–109∘109^\circ. That’s a few degrees below the tetrahedral ideal.

  3. Why the C–O–C angle in ethers is greater than 109.5°

    Now both substituents on oxygen are alkyl groups (R and R′). These are much bulkier than a hydrogen atom. The two alkyl groups repel each other sterically — they try to get as far apart as possible. This steric repulsion overcomes the lone‑pair compression effect and forces the C–O–C angle to open up.

    Tip

    Think of it like two people trying to stand close together while each is carrying a large backpack — they naturally lean away from each other. …

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