[sirpsychosexy]

(CH3)3CCH2CH2CH(CH3)CH(CH3)2

is this named 2,3,6,6-tetramethylheptane

or

2,2,5,6-tetramethylheptane


thanks for any help on this

[madscientist]

What you have to look at is the longest possible carbon chain in the formula,

(CH3)3CCH2CH2CH(CH3)CH(CH3)2


7 in the formula you have given, with methyl substituents at carbons 2,2,5 and 6. so your second guess was right.

[sirpsychosexy]

thanks for the quick reply, but i dont see why it is 2,2,5,6

 ... so there's substituents on the 2 carbon from both ends, and the next substituent is on the 3 carbon, where the other way doesnt offer another subst. until the 5 carbon.... i thought you number the mother chain in the manner that offers the most substituents at lower numbers?


thanks again   

[AWK]

http://www.cem.msu.edu/~reusch/VirtualText/nomexmp1.htm
http://www.acdlabs.com/iupac/nomenclature/79/r79_36.htm

[madscientist]

If you number the carbon chain from the other end i.e from right to left in the structure I posted you end up with 2,3,6,6-tetramethylheptane which has more substituents at higher numbers than if you number it from left to right which gives 2,2,5,6-tetramethylheptane, make sense?

[sirpsychosexy]

...however the second substituent going right to left come on carbon 3, where as the second substituent going left to right comes on carbon 5....

im still a little confused on this part


thanks for your help

[madscientist]

overall it gives lower numbered substituents, I think if you total all the numbers i.e 2+3+6+6 = 17 for right to left numbering and 2+2+5+6 = 15 for left to right numbering. Read the sites that AWK posted, they pretty much hand you your answer on a silver platter.

[AWK]

n excert from the cesond link:

Rule 2.2 - The longest chain is numbered from one end to the other by Arabic numerals, the direction being so chosen as to give the lowest numbers possible to the side chains. When series of locants containing the same number of terms are compared term by term, that series is "lowest" which contains the lowest number on the occasion of the first difference. This principle is applied irrespective of the nature of the substituents.