# Why is the Carbon always Delta positive in mechanisms?

Specifically in A-level, In ANY case has/will the carbon ever be delta negative. If so why or why not.
Original post by moali158
Specifically in A-level, In ANY case has/will the carbon ever be delta negative. If so why or why not.

There are cases in A level where carbon could assume a δ- charge in a mechanism, though iirc none of the mechanisms you are taught involve such carbons (they are either δ+, or have a formal +1 or -1 charge), but it is the unfamiliar mechanisms you could be expected to work out in an exam that may involve them.

To establish whether the carbon has a δ+ or δ- charge, you need to consider what it is bound to and if the things it is bound to are more or less electronegative than carbon. Halogens, oxygen and nitrogen are more electronegative than carbon and (most) metals are less electronegative than carbon.
(edited 4 months ago)
Original post by TypicalNerd
There are cases in A level where carbon could assume a δ- charge in a mechanism, though iirc none of the mechanisms you are taught involve such carbons (they are either δ+, or have a formal +1 or -1 charge), but it is the unfamiliar mechanisms you could be expected to work out in an exam that may involve them.

To establish whether the carbon has a δ+ or δ- charge, you need to consider what it is bound to and if the things it is bound to are more or less electronegative than carbon. Halogens, oxygen and nitrogen are more electronegative than carbon and (most) metals are less electronegative than carbon.

So if its a carbon-oxygen/halogen/nitrogen bond the carbon would be delta negative in this scenario? In that case, how would i approach the mechanism and would it be any different?
Original post by moali158
So if its a carbon-oxygen/halogen/nitrogen bond the carbon would be delta negative in this scenario? In that case, how would i approach the mechanism and would it be any different?

Carbon would be δ+ if it is bound to a halogen, oxygen or nitrogen as these elements would pull electrons away from the carbon. Carbon would only be δ- in cases where it is bonded to a metal (afaik, only Edexcel really has any instances where this is the case).

The mechanisms should show the movements of electrons to positive charges (so either formal + charges or δ+), so you need to identify possible areas where electrons could be - they will either be lone pairs or within bonds.

I think for your benefit, it is worth finding examples of each of the mechanisms you definitely need to know for your exam board and save my exams does a good job of this imo.
(edited 4 months ago)

Carbon would take a δ- charge if covalently bonded to a less electronegative species.

So for instance, if you had a:
C-B bond C δ- B δ+
C-P bond C δ- P δ+
C-S bond Non-polar
C-N bond N δ- C δ+

However as you said, at A Level, you're unlikely to encounter a molecule with a C δ- charge. There are also considerations with surrounding atoms, e.g. If a carbon atom was bonded to both Boron and Oxygen - however, you're still unlikely to encounter that scenario.

Hope this helps!
Original post by dbhc2411

Carbon would take a δ- charge if covalently bonded to a less electronegative species.

So for instance, if you had a:
C-B bond C δ- B δ+
C-P bond C δ- P δ+
C-S bond Non-polar
C-N bond N δ- C δ+

However as you said, at A Level, you're unlikely to encounter a molecule with a C δ- charge. There are also considerations with surrounding atoms, e.g. If a carbon atom was bonded to both Boron and Oxygen - however, you're still unlikely to encounter that scenario.

Hope this helps!

Alright, Thank you! I think it would be best for me to just ignore this as a whole and invest my concerns in other areas 😅
Original post by moali158
Alright, Thank you! I think it would be best for me to just ignore this as a whole and invest my concerns in other areas 😅

Honestly I think it's really ace that you're curious about these things, because it helps you to cement your knowledge of chemistry!