Determining an unknown Hydrocarbon X

[miniradman]

Homework Statement

A gaseous hydrocarbon X had an approximate molecular weight of 57. It reacted with both bromine and dilute permanganate solutions. Give the structural formula of the only 3 compounds consistent with this information.
X was treated with hydrogen bromide, and the resulting liquid was then boiled with with alcoholic sodium hydroxide solution. Only one compound was formed. Deduce the structure of X, giving your reasoning.

2. The attempt at a solution
Hello all.

I'm having trouble finding a single structure which satisfies all the criteria.

The first pieces of data I'm given is that the molecular weight is a butene of some description (has a MW of 58 and reacts with permanganate and bromine). So hence my three possible structures would be; 1-butene, 2-butene and 2-methylprop-1-ene.

However, I'm finding it hard to distinguish these three with the next pieces of information we're given. I think that with the given processes involved, the result will be an alcohol of some description, however I cannot think of a mechanism which will break the double bond of the product.

I'm assuming, where it says, one product is formed is the key here which I'm guessing means there are no major and minor products involved.

 

[Borek]

57 is not 58.

 

[DrClaude]

The first pieces of data I'm given is that the molecular weight is a butene of some description (has a MW of 58 and reacts with permanganate and bromine).

I think you made a typo in the molecular weight.

However, I'm finding it hard to distinguish these three with the next pieces of information we're given. I think that with the given processes involved, the result will be an alcohol of some description, however I cannot think of a mechanism which will break the double bond of the product.

Check out http://www.masterorganicchemistry.com/reaction-guide/addition-of-hbr-to-alkenes/

I'm assuming, where it says, one product is formed is the key here which I'm guessing means there are no major and minor products involved.

Exactly. The reactions shoud result in a single product with no other possibility.

 

[miniradman]

Whoops, sorry the MW would be 56. I know that 56 does not equal 57, however I cannot think of a hydrocarbon with a MW of exactly 57.

 

[Borek]

Sorry, missed the error, what I meant was that you should look for a hydrocarbon with exactly given molar mass.

Do you know what degree of unsaturation is? Do you know what it tells about the molecule? Does it require a double or triple bond?

 

[DrClaude]

Sorry, missed the error, what I meant was that you should look for a hydrocarbon with exactly given molar mass.

But the statement of the problem says "approximate molecular weight of 57." Personally, I would take that to be in the range 56 to 58.

 

[Borek]

C₄H₈ is 56.11, I took "approximate" to mean "ignore the part after the decimal point".

 

[DrClaude]

I took "approximate" to mean "ignore the part after the decimal point".

I guess I'm sloppier than you are :tongue2: