Definition: A secondary hydrogen is the hydrogen attached to a secondary carbon, i.e., a carbon bonded to exactly two other carbon atoms.
Now, count the secondary hydrogens present in each alkane.
2,2,3,3-TetramethylpentaneStructure:
CH3​−C(CH3​)2​−C(CH3​)2​−CH2​−CH3​Here
C2 and
C3 are quaternary carbons, and only
C4 is a secondary
CH2​ carbon.
Number of secondary hydrogens
=2.
2,2,4,4-TetramethylhexaneStructure:
CH3​−C(CH3​)2​−CH2​−C(CH3​)2​−CH2​−CH3​Here
C3 and
C5 are secondary
CH2​ carbons, each carrying
2 hydrogens.
Number of secondary hydrogens
=2+2=4.
4-Ethyl-3,4-dimethyloctaneThe secondary carbons are
C2,
C5,
C6,
C7 of the main chain and the
CH2​ carbon of the ethyl group.
Number of secondary hydrogens
=5×2=10.
2,2,4,5-TetramethylheptaneStructure:
CH3​−C(CH3​)2​−CH2​−CH(CH3​)−CH(CH3​)−CH2​−CH3​Here
C3 and
C6 are secondary
CH2​ carbons, each carrying
2 hydrogens.
Number of secondary hydrogens
=2+2=4.
Conclusion: Only 2,2,3,3-tetramethylpentane contains exactly two secondary hydrogens.
Hence, the correct answer is 2,2,3,3-tetramethylpentane (first option).
