JEE Advanced 2023 Paper 1 · Chemistry · Carbonyl Compounds, Carboxylic Acids and Amines · medium

Acetone is treated with two equivalents of benzaldehyde in the presence of dilute aqueous NaOH to give the conjugated ketone P, C17H14O…

Numerical answer

Acetone is treated with two equivalents of benzaldehyde in the presence of dilute aqueous

\mathrm{NaOH}

to give the conjugated ketone

\mathbf{P}

\mathrm{C_{17}H_{14}O}

(dibenzylideneacetone), formed by two successive aldol condensations. Let

m

be the total number of geometrical (configurational) isomers possible for

\mathbf{P}

and

n

the number of distinct

^{13}\mathrm{C}

NMR signals expected for the

(E,E)

isomer. The value of

n-m

Show answer and solution

Answer: 4

Structure:

\mathrm{PhCH{=}CH\!-\!CO\!-\!CH{=}CHPh}

(formed by dehydration after each aldol step). Geometrical isomers: each

\mathrm{C{=}C}

can be

E

Z

, giving

(E,E)

(Z,Z)

and

(E,Z)

; the

(Z,E)

form is identical to

(E,Z)

because the molecule is symmetric about the carbonyl. So

m=3

^{13}\mathrm{C}

signals for the

(E,E)

isomer, which has a

C_2

/mirror symmetry making the two halves equivalent: carbonyl carbon (1),

\alpha

-vinyl

\mathrm{CH}

(1),

\beta

-vinyl

\mathrm{CH}

(1), ipso aromatic carbon (1), ortho carbons (1), meta carbons (1), para carbon (1). So

n=7

. Hence

n-m=7-3=4

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