carboxylic acids GCSE help

hey guys! i'm just doing some chem revision for my paper 2 chem on Monday ( i do AQA higher triple).

i'm confused about the structure of carboxylic acids.
i just got this question saying 'tick the diagram which you think looks like propanoic acid'.

i googled what the general formula for CA is and it states that it is:
Cn H2n+1 COOH

iim just really confused on how i should know what propanoic acid would look like compared to (for example) butanoic acid.

i get that all carboxylic acids have the functional group COOH but apart from that, i couldn't tell butanoic acid and propanoic acid apart.

any help??

i'll try and see if i can attach the question

many thanks :smile:

edit: i was able to attach the question and the answer is C btw

(edited 1 year ago)

Screenshot 2022-06-18 at 21.02.41.png53.0KB

Reply 1

Ira Acedia

The general formula is correct, but you should start with n=0, not n=1, like I assume you are doing.

When n=0, you have methanoic acid.
n = 1: ethanoic acid
n = 2: propanoic acid

etc etc. Therefore, when looking at the compound's structure, merely count the number of carbons in the longest carbon chain with the carboxylic acid group -- this is the what you'll use to name it. For option A, 1 carbon means it's meth. B has 2 carbons, so it's eth. C has 3 carbons so it's prop. D has 4 carbons so it's but.

Reply 2

bears15

You can identify them by the amount of carbon atoms in it. I remember it through this mnemonic:

Monkeys = Meth.. (1 carbon)
Eat = Eth... (2 carbons)
Peanut = Prop... (3 carbons)
Butter = But... (4 carbons)
5 carbons and above have their respective name eg. pent..., hex..., hept..., oct... etc.

For carboxylic acids however, methanoic acid starts at 0 (since there's already a carbon in the COOH).
Hope this helps. Good luck for chemistry. :smile:

(edited 1 year ago)

Reply 3

thrivingfrog

Chat Forum Helper, Study Forum Helper

Original post by harlz_chalamet

edit: i was able to attach the question and the answer is C btw

The formula for propanoic acid is CH3CH2COOH and the formula for butanoic acid is CH3CH2CH2COOH so you'd use that to work out which one the displayed formula.

This is what I do anyway- sorry if I've confused you more!

Reply 4

harlz_chalamet

Original post by thrivingfrog

yo bro

i'm kinda confused because other people in the comments are saying that the formula is Cn H2n+1 COOH

did you just memorise the formula for one CA and then the others just follow the pattern??

Reply 5

tony_dolby

"i get that all carboxylic acids have the functional group COOH but apart from that, i couldn't tell butanoic acid and propanoic acid apart.'

You've understood the hardest part. All you need to do now is number the carbons (literally, write numbers above each carbon). The COOH carbon is always number one. Keep numbering until you reach the end of the carbon chain. If you have three carbons the molecule is propanoic acid. If you have four, the molecule is butanoic acid.

(edited 1 year ago)

Reply 6

tony_dolby

Original post by harlz_chalamet

Personally I think the general formula method is overly complicated. If you see 'COOH' you're thinking 'carboxylic acid'. Try the numbering method above and see if this helps.

Reply 7

thrivingfrog

Chat Forum Helper, Study Forum Helper

Original post by harlz_chalamet

Both versions work, this is just the method I use.
Methanoic acid is HCOOH
Ethanoic acid CH3COOH
Propanoic acid CH3CH2COOH
and then you just continue adding CH2 as you go up.

Reply 8

Ira Acedia

Original post by tony_dolby

Personally I think the general formula method is overly complicated. If you see 'COOH' you're thinking 'carboxylic acid'. Try the numbering method above and see if this helps.

I agree for the most part, though I have seen questions that ask you to state the general formula of certain homologous series (granted, A-level questions but they're GCSE difficulty so wouldn't be surprised if some specs had them), so it's good to know but I personally rarely use it.