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NEET CHEMISTRYEasy

The linear molecule among the following is:

A

CH3−C≡C−CH3CH_3-C \equiv C-CH_3

B

CH2=CH−CH2−C≡CHCH_2=CH-CH_2-C \equiv CH

C

CH3−CH2−CH2−CH3CH_3-CH_2-CH_2-CH_3

D

CH3−CH=CH−CH3CH_3-CH=CH-CH_3

Step-by-Step Solution

  1. Hybridisation and Geometry: The shape of an organic molecule is determined by the hybridisation of its carbon atoms [NCERT 11th, Ch 12, Sec 12.3].
  • sp hybridisation: Bond angle = 180° (Linear).
  • sp² hybridisation: Bond angle = 120° (Trigonal Planar).
  • sp³ hybridisation: Bond angle = 109.5° (Tetrahedral).
  1. Analyze Option A (CH3−C≡C−CH3CH_3-C \equiv C-CH_3, 2-Butyne):
  • The central carbons (C2C_2 and C3C_3) are triply bonded. Triply bonded carbons are sp hybridised. This creates a linear arrangement of atoms along the C1−C2−C3−C4C_1-C_2-C_3-C_4 axis (180∘180^\circ bond angle).
  • While the methyl hydrogens are tetrahedral, the carbon skeleton itself is linear [NCERT 11th, Ch 13, Sec 13.4.2].
  1. Analyze Other Options:
  • Option B (CH2=CH−CH2−C≡CHCH_2=CH-CH_2-C \equiv CH): Contains sp2sp^2 (trigonal planar) and sp3sp^3 (tetrahedral) carbons, creating a bent/zigzag chain.
  • Option C (CH3−CH2−CH2−CH3CH_3-CH_2-CH_2-CH_3, n-Butane): All carbons are sp3sp^3 hybridised. The chain is zigzag (tetrahedral geometry at each carbon).
  • Option D (CH3−CH=CH−CH3CH_3-CH=CH-CH_3, 2-Butene): Contains sp2sp^2 carbons. The geometry around the double bond is trigonal planar, not linear.
  1. Conclusion: 2-Butyne is the linear molecule.
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