Nomenclature - can someone explain the name of this haloalkane?

Watch
Tolgash
Badges: 19
Rep:
?
#1
Report Thread starter 2 years ago
#1
Hello chemists of TSR, I am confused by the answer to an organic chemistry question for the IUPAC name of an organic compound (the question is in the link below):

https://imgur.com/a/S7PM1jS

I believe the name of the compound is 2-ethyl-4-methyl-2,4-dichloropentane.


I say this because the longest side chain is on the right, there is one
C2H5 (methyl) on the second chain and one CH3 (ethyl) on the fourth chain, with two chlorides on the second and fourth chains. There are also five carbons on the main chain, hence the prefix pent-.

However, according to the mark scheme, the name is actually 2,4-dichloro-2,4-dimethylhexane.

I don't understand how this is correct. First, aren't there five carbon molecules in the main chain, not six? How is the prefix hex- when there are only five? Secondly, isn't there only one methyl? Those two side chains that aren't halogen ions look awfully different. How can they have the same name?

I think I've overlooked something or don't understand how to apply the IUPAC rules for naming organic compounds yet. Regardless of which it is, any help would be much appreciated.
0
reply
AstyPasty5
Badges: 5
Rep:
?
#2
Report 2 years ago
#2
(Original post by Tolgarda)
Hello chemists of TSR, I am confused by the answer to an organic chemistry question for the IUPAC name of an organic compound (the question is in the link below):

https://imgur.com/a/S7PM1jS

I believe the name of the compound is 2-ethyl-4-methyl-2,4-dichloropentane.


I say this because the longest side chain is on the right, there is one
C2H5 (methyl) on the second chain and one CH3 (ethyl) on the fourth chain, with two chlorides on the second and fourth chains. There are also five carbons on the main chain, hence the prefix pent-.

However, according to the mark scheme, the name is actually 2,4-dichloro-2,4-dimethylhexane.

I don't understand how this is correct. First, aren't there five carbon molecules in the main chain, not six? How is the prefix hex- when there are only five? Secondly, isn't there only one methyl? Those two side chains that aren't halogen ions look awfully different. How can they have the same name?

I think I've overlooked something or don't understand how to apply the IUPAC rules for naming organic compounds yet. Regardless of which it is, any help would be much appreciated.
The mark scheme is (unsuprisingly) correct. I think where you've got confused is that the longest chain of carbon molecules includes the 'ethyl' group that you mention, causing it to have six carbons and therfore be hexane. Despite the way that the shape is drawn with the 5th and 6th carbon being at a right angle to the rest of the chain, these carbons are still part of the longest chain which is therefore hexane.

This leaves the both chlorines and the methyl groups on the 2nd and 4th carbons of the chain, leading the compound to be called 2,4-dichloro-2,4-dimethylhexane
3
reply
_NMcC_
Badges: 15
Rep:
?
#3
Report 2 years ago
#3
Yes agreed with the above answer.

Just be very careful when counting carbons as the seemingly separate 'ethyl group' attached (facing down) to the third carbon on right of the molecule can be counted as part of the longest chain. Start counting from the bottom ethyl to the main 'chain' (that's 3 carbons) then go left and pick any one of the two methyls - you will get 6 in total.

It seems arranged in a way that's designed to confuse, so don't worry too much, it just takes practice.

Then you can determine side-chains and their naming order (alphabetical - i.e Chloro before Methyl).
Last edited by _NMcC_; 2 years ago
1
reply
tesconyc
Badges: 18
Rep:
?
#4
Report 2 years ago
#4
I hate chemistry. I'm so bad at it. respected to tsr chemist s
0
reply
Tolgash
Badges: 19
Rep:
?
#5
Report Thread starter 2 years ago
#5
(Original post by AstyPasty5)
The mark scheme is (unsuprisingly) correct. I think where you've got confused is that the longest chain of carbon molecules includes the 'ethyl' group that you mention, causing it to have six carbons and therfore be hexane. Despite the way that the shape is drawn with the 5th and 6th carbon being at a right angle to the rest of the chain, these carbons are still part of the longest chain which is therefore hexane.

This leaves the both chlorines and the methyl groups on the 2nd and 4th carbons of the chain, leading the compound to be called 2,4-dichloro-2,4-dimethylhexane
Ah, I understand. Thanks for clearing that one up. I do, however, have another question lingering in my head from this. How can I tell whether one of those chains on the side is actually part of the main chain and not something like an ethyl group?
0
reply
Tolgash
Badges: 19
Rep:
?
#6
Report Thread starter 2 years ago
#6
(Original post by tesconyc)
I hate chemistry. I'm so bad at it. respected to tsr chemist s
Same man.
0
reply
AstyPasty5
Badges: 5
Rep:
?
#7
Report 2 years ago
#7
(Original post by Tolgarda)
Ah, I understand. Thanks for clearing that one up. I do, however, have another question lingering in my head from this. How can I tell whether one of those chains on the side is actually part of the main chain and not something like an ethyl group?
I think the only real way that you can tell whether alkyl groups are part of the main chain is by just counting the number of carbons and comparing the lengths of the chains formed. I don't think there is some easy and efficient trick, but practice makes this process much faster and much more efficient
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Back
to top
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

Do you think receiving Teacher Assessed Grades will impact your future?

I'm worried it will negatively impact me getting into university/college (189)
43.65%
I'm worried that I’m not academically prepared for the next stage in my educational journey (49)
11.32%
I'm worried it will impact my future career (33)
7.62%
I'm worried that my grades will be seen as ‘lesser’ because I didn’t take exams (91)
21.02%
I don’t think that receiving these grades will impact my future (46)
10.62%
I think that receiving these grades will affect me in another way (let us know in the discussion!) (25)
5.77%

Watched Threads

View All