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Collinear carbons in X: double acetylide alkylation of buta-1,3-diyne with trans-crotyl bromide

Buta-1,3-diyne, two equivalents of NaNH2 and excess trans-crotyl bromide, watched at two levels at once: the liquid-ammonia flask on the left, the molecules on the right, in step. Curved arrows move each electron pair as NH2- pulls off both acidic sp C-H protons and each acetylide carbanion attacks a CH2-Br from the back (SN2, Walden umbrella flip), while the grey NaNH2 is used up and white NaBr settles. X is then relaxed to its real angles (sp 180, sp2 120, sp3 109.5 degrees) and spun about its axis: the carbons that never move are counted. Products come from pushing the arrows on molecule graphs; the count is made twice, by rule and on a geometric model. No prediction stage - input, run, watch, result.

Sub-topics covered

• Acidity of terminal alkynes (sp C-H, pKa about 25)
• Sodium amide in liquid ammonia
• Acetylide ions as nucleophiles: SN2 on primary halides
• Hybridisation and bond angles: sp, sp2, sp3
• Counting collinear atoms in a molecule

What it teaches

• Two equivalents of NaNH2 remove both terminal sp C-H protons of buta-1,3-diyne, giving the acetylide dianion
• Each acetylide attacks the CH2 of trans-crotyl bromide by SN2 (backside attack, one step); the trans C=C is untouched
• X = CH3CH=CHCH2-C#C-C#C-CH2CH=CHCH3 (C12H14), with 4 sp carbons in the middle
• An sp carbon holds its two neighbours at 180 degrees, so the four sp carbons and the two CH2 carbons bonded to them lie on one line
• The CH2 is sp3 (109.5 degrees) and the next carbon is sp2, so the line stops there: 6

Simulation ID

• ADV-2026-P1-CHE-Q12 · revision 1

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