Isomers help

Compound I and J have molecular formula C5H12O

I cannot be oxidised by potassium dichromate but can be dehydrated to form an alkene

J can be oxidised by acidified potassium dichromate but cannot be dehydrated to form an alkene


So I has to be tertiary alcohol?

J has to be primary or secondary

I dont get the dehydration part if someone can explain than.

Compound I

Yes so Tertiary alcohols don't oxidise but you can dehydrate them via the E1 mechanism to form an alkene. You are essentially removing water from the compound.

. Think of the OH coming from the hydroxyl group.
. The H from a carbon adjacent to the C-OH group (called a Beta Hydrogen) - hence why it's called dehydration, removal of H20.

It's easier to understand if you see it, like this example:


Note: Ignore the H30+ and HSO4-, most A level Specs only require you to have H20 as a product, this is more advanced. The reasons why a Proton is added to the hydroxyl are a bit too deep for A level - however if you are interested, read around Leaving groups.

Just understand that H20 is removed from the compound during dehydration.

You can see the removal of the Beta Hydrogen in the 3rd step in the diagram (closest to the right). This is ultimately what forms the alkene.

It's called Beta because it's attached to the second carbon from this functional group (the Beta carbon, the functional group is the hydroxyl).

The Carbon attached to the hydroxyl group is called the Alpha carbon because it's directly attached to it (the first carbon attached to the functional group). It's a useful naming convention when you are trying to describe a reaction - you don't need to know it for A level though.

Most A level specs don't require the dehydration mechanism as it's quite advanced however if you want to improve your understanding to a deeper level, the full mechanism is here:

https://chem.libretexts.org/Core/Organic_Chemistry/Alkenes/Synthesis_of_Alkenes/Alkenes_from_Dehydration_of_Alcohols



Compound J

What is the only other type of functional group you know that can be oxidised to something else but can't be dehydrated?

If you imagine that heating ethene and water vapour together gives you ethanol (breaks the double bond, adds the -OH group and the H atom), dehydration is literally the opposite reaction. You take the -OH group and H atom away, they combine to form water, and the 'spare' electron on the C atom that was bonded to the -OH group forms a double bond with its neighbouring C atom.

Im massively confused, is dehydration elimination reaction?
Does dehydration work for all types of alcohols? or does it have to be tertiary, can you dehydrate a primary alcohol?

Sorry for the questions, I just don't get it

Im massively confused, is dehydration elimination reaction?
Does dehydration work for all types of alcohols? or does it have to be tertiary, can you dehydrate a primary alcohol?

Sorry for the questions, I just don't get it

Yes, Dehydration is always elimination, as you are eliminating Water (H20 formed from OH and H+).

You can dehydrate any alcohol you like, the mechanisms are different though.

Primary = E2
Secondary/Tertiary = E1

Are there different reactants for each? Like I read H3PO4 is used and somewhere else H2SO4

Anyway the reason im confused is from the markscheme, both the answers are alcohols? So when it says one cannot be dehydrated what is that about?

Structure J is a primary alcohol as it turns out, interesting. I originally suspected it was an aldehyde, although looking more closely at the formula I realised it could never be an Aldehyde! Confused me as well!

It must be your exam board is taking primary alcohols as functional groups that can't be dehydrated - maybe because of the weird E2 mechanism.

Take it as that then.

Primary Alcohols don't dehydrate (although that's actually wrong).

Secondary and Tertiary do dehydrate.

You can dehydrate ethanol right?
Do you use H3po4?