Answer the following: i) Haloalkanes easily dissolve in organic solvents. Why? ii) What is a racemic mixture? Give an example. iii) Of the two bromo derivatives, C$_6$H$_5$CH(CH$_3$)Br and C$_6$H$_5$CH(C$_6$H$_5$)Br, which one is more reactive towards the S$_\mathrm{N}$1 substitution reaction, and why?

(i) Similar (weak van der Waals) intermolecular forces between haloalkanes and organic solvents. (ii) An equimolar mixture of two enantiomers, optically inactive by external compensation — e.g. (±)-butan-2-ol. (iii) C₆H₅CH(C₆H₅)Br — its carbocation is stabilised by resonance with two phenyl rings.

CBSE Class 12 · 2011 · Delhi · Haloalkanes and HaloarenesShort Answer

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Answer the following:
i) Haloalkanes easily dissolve in organic solvents. Why?
ii) What is a racemic mixture? Give an example.
iii) Of the two bromo derivatives, C

_{6}

_{5}

CH(CH

_{3}

)Br and C

_{6}

_{5}

CH(C

_{6}

_{5}

)Br, which one is more reactive towards the S

_{N}

1 substitution reaction, and why?

Solution

(i) The new attractions set up between haloalkane and organic-solvent molecules are of the same (van der Waals) kind and strength as those broken — so haloalkanes dissolve readily in organic solvents (and not in water, whose H-bonds they cannot replace).
(ii) A racemic mixture (±) contains the two enantiomers in equal amounts; their equal and opposite rotations cancel (external compensation), so it is optically inactive. Example: (±)-butan-2-ol.
(iii) \textbf{C

_{6}

_{5}

CH(C

_{6}

_{5}

)Br}: its ionisation gives the carbocation C

_{6}

_{5}

\overset{+}{C}HC

_{6}

_{5}

, resonance-stabilised by \textit{two} phenyl rings — more stable than C

_{6}

_{5}

\overset{+}{C}HCH

_{3}

(one ring) — so it is faster in S

_{N}

CBSE Class 12 Board · 2011 · Delhi · Subjective

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CBSE Class 12 · 2011 · Delhi · Haloalkanes and HaloarenesShort Answer

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