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OCR Alcohols - oxidation and sterioisomers, help?

Can someone please help me with this question?? Part ii?

I thought when you heated a secondary alcohol you get a ketone?

It is the January 2012 paper, if you want to look at the markscheme.

Reply 1

herothing11

When you heat a secondary alcohol with acidified potassium dichromate you get a ketone,
Here the alcohol is being reacted with concentrated H2SO4, which is a dehydrating agent. They have actually told you at the top of the question that it eliminates water from an alcohol.
Therefore if you eliminate water from a secondary alcohol you will get an alkene, rather than a ketone.

Does that make sense?

Reply 2

myah_94

Yes they are, but when heated with acidified dichromate ions only. However, when heated with an acid like that in the question, it becomes an elimination reaction called dehydration, where two hydrogens and an oxygen is removed
Hope this helps

(edited 10 years ago)

Reply 3

BJack

Original post by Saif95

Can someone please help me with this question?? Part ii?

I thought when you heated a secondary alcohol you get a ketone?

It is the January 2012 paper, if you want to look at the markscheme.

H₂SO₄ can act as a dehydrating agent. Chemguide has an explanation of what's going on mechanistically. You should find there are only three different things you can get by eliminating water from this molecule. (Remember that the final alkene could be cis or trans.)

Reply 4

dr-jimmy

Original post by Saif95

Can someone please help me with this question?? Part ii?

I thought when you heated a secondary alcohol you get a ketone?

It is the January 2012 paper, if you want to look at the markscheme.

As others have said it's a dehydration reaction. Three isomers can be made because the double bond forms between the carbon with the hydroxyl group on it and an adjacent carbon, so the c=c bond can form between carbon one and carbon two or carbon two and carbon three. This means two stereoisomers can be made with the c=c bond between carbon two and three, and one structural isomer when the c=c bond is between carbon one and two. :smile: