In this reaction, we assumed the limiting reagent was styrene, but this may not

Question

In this reaction, we assumed the limiting reagent was styrene, but this may not be accurate. What could have happened that would result in a reagent other than styrene being the limiting reagent? (Hunt: did you add BH3 to the reaction?) OH a. BH3, THF b. H202, NaOH (aq) OH Phenylethyl alcohol anti-Markovnikov -Methy!benzyl alcohol Markovnikov Styrene Preparation of BH3: THF 2 NaBH4 16 mmoles (Ola 15) 12 6.7 mmoles 479) 2 BH3 2H2(g) I .7 The hydroboration step: THF 3 PhCH=CH2 20 mmoles tyrene BH3 + major isomer C2.imiting The oxidation step: THF (PhCH2CH2)33 NaOH 3 H202 3 PhCH2CH20H Na3BO3 3 H20

Explanation / Answer

we have 16 mmole NaBH4 and 6.7 mmole I2.

2 mole NaBH4 react with one mole I2 to form 2 mole BH3.

so here NaBH4 remains in excess and I2 is the limiting reactant in prepareation of BH3 reaction.

so mmole of BH3 produce = 6.7 * 2 = 13.4 mmole.

In the hydroboration step, we have 20 mmole styrene.

here in this reaction one mole BH3 react with 3 mole styrene to form one mole product(major isomer).

so in this reaction styrene is the limiting reactant because there are 13.4 mmole BH3 and styrene is 20 mmole.

so styrene being the limiting reagent.

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