Question 1

Why does trans-but-2-ene with Br₂ give the meso product, not (±)?

Accepted Answer

Br₂ adds anti (the bromonium ion forces Br⁻ to attack from the opposite face). With a trans alkene, anti addition places the two Br atoms so the product has an internal centre of symmetry — that is the meso-2,3-dibromobutane. The (±) pair would come from cis-but-2-ene under the same anti addition.

Question 2

What is the meso vs (±) rule for addition reactions?

Accepted Answer

Change either the alkene geometry (cis/trans) or the addition mode (syn/anti) and the answer flips between meso and (±); change both and it stays the same. Concretely: anti addition gives meso from trans and (±) from cis, while syn addition gives meso from cis and (±) from trans.

Question 3

Why can you only see the meso symmetry in the anti conformation?

Accepted Answer

meso-ness is a property of configuration, but the centre of symmetry only becomes visually obvious in the anti (180°) Newman projection. In a skew or gauche conformation the symmetry is hidden, so you must rotate the back carbon to anti before checking. NEET 2025 Q51 deliberately drew the glycol skewed to exploit this.

Question 4

Is a meso compound optically active?

Accepted Answer

No. A meso compound has stereocentres but also an internal plane or centre of symmetry, so its mirror image is superimposable on itself. The rotations from its two halves cancel internally, making it optically inactive (achiral) despite having chiral centres.

Question 5

How does cold alkaline KMnO₄ add to a double bond?

Accepted Answer

Cold dilute alkaline KMnO₄ does syn dihydroxylation: it forms a cyclic manganate ester across one face of the C=C, so both –OH groups end up on the same face. With cis-but-2-ene this syn addition yields the meso diol; with trans-but-2-ene it yields the (±) pair.

meso vs (±): the stereochemistry of addition

Practice

Learned it? Now prove it under the clock.