Chemistry · Haloalkanes And Haloarenes · NEET
SN2 always gives inversion of configuration. The nucleophile attacks the carbon from the side directly opposite the leaving group (the back side). Because the leaving group is on one side, the incoming nucleophile is forced to the other side. The three other bonds then flip over, like an umbrella turning inside out in strong wind. NCERT example: (-)-2-bromooctane reacts with NaOH to give (+)-octan-2-ol, where the -OH sits opposite to where the Br was. This flip is called Walden inversion.
In SN1 the leaving group leaves first, making a flat (planar), sp2 hybridised carbocation. A flat carbocation has no left or right preference. So the nucleophile can attack from the top face or the bottom face with equal chance. Attack from one face keeps the old arrangement; attack from the other face inverts it. This gives a roughly 50:50 mix of the two mirror images (enantiomers). A 50:50 mix is a racemic mixture, so we call it racemisation.
Walden inversion is the flipping of the spatial arrangement (configuration) at a chiral carbon during backside attack in an SN2 reaction. Think of an umbrella that turns inside out. The reactant and product are on opposite mirror-image forms. Important: the sign of rotation (+ or -) does not always match the label. NCERT shows (-)-2-bromooctane giving (+)-octan-2-ol, but the product still has the inverted configuration even though its sign changed to plus.
A racemic mixture has equal amounts of the dextro (+) form and the laevo (-) form. One form rotates plane polarised light to the right, the other rotates it by the same amount to the left. The two rotations cancel out. The net rotation becomes zero, so the mixture is optically inactive. This is why NCERT writes the SN1 hydrolysis product of 2-bromobutane as (±)-butan-2-ol, where (±) means racemic.
Retention means the product keeps the same spatial arrangement as the reactant (no bond to the chiral carbon is broken). Inversion means the arrangement flips to the mirror form; SN2 does this. Racemisation means you get a 50:50 mix of both mirror forms (one retained, one inverted); SN1 does this and the product is optically inactive. For NEET, remember: SN2 = inversion, SN1 = racemisation, and reactions where no bond to the chiral carbon breaks = retention.
In pure theory SN1 gives 100% racemisation (perfect 50:50). But NCERT and NEET treat SN1 as giving racemisation, so for the exam pick racemisation. In real experiments there is often slightly more inversion than retention, because the leaving group can shield one face for a moment. For NEET answers, always choose racemisation for SN1 unless the question specifically discusses partial inversion.
The incorrect statement regarding chirality is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
SN2 gives inversion of configuration because the nucleophile attacks from the back side (opposite the leaving group), flipping the molecule. SN1 gives racemisation, not inversion.
The (±) sign means the product is a racemic mixture, an equal 50:50 mix of the (+) and (-) enantiomers. It is optically inactive. NCERT writes SN1 hydrolysis of 2-bromobutane as giving (±)-butan-2-ol.
Yes. NCERT clearly states the sign of optical rotation is not always tied to the actual configuration. Two compounds with the same configuration can rotate light in opposite directions, so a + can become a - even when configuration is retained.
Tertiary halides prefer the SN1 path because they form a stable carbocation. Since SN1 goes through a flat carbocation attacked from both faces, tertiary chiral halides usually give racemisation.
NEET regularly asks statement-based questions linking SN1 to racemisation and SN2 to inversion (for example NEET 2022). Knowing these two links and the meaning of racemic mixture lets you answer quickly without long calculation.
