I'm teaching myself organic chemistry and am struggling with differentiating between dimethyl and ethyl substituent groups. Example: tert-butyl (1,1-dimethylethyl)

This chapter follows the one on bond-line structures, and I realize one of the rules for making a valid bond-line structure for straight-chain alkanes involves zig-zagging carbon. But I am confused on how to represent the substituent groups - is the ethyl group the V, and the methyl groups are each of the branches from the middle carbon atom of the substituent group?

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  • $\begingroup$ Just the opposite of your color scheme, the ethyl group is red and the dimethyl (meaning 2 methyl groups) are your 2 blue lines. BTW, we typically draw hydrocarbon structures using zig-zag lines like you've done above, but these are not resonance structures. $\endgroup$ – ron May 10 '15 at 18:28
  • $\begingroup$ Sorry, I meant bond-line structures. How do we know the ethyl group is the straight line and the two methyl groups are the 'branches'? (without being given the name of the substituent group) $\endgroup$ – imaginov May 10 '15 at 18:34
  • $\begingroup$ the longest straight chain (coming off the backbone of your main chain) in your example turns out to be 2 carbons or ethyl. You can select it 3 different ways (center red line, bottom of center red line plus right blue line, bottom of center red line plus left blue line) but it doesn't matter, in this case you get the same result. Whichever of the three 2 carbon fragments you select, it has two methyl groups branching off of it. $\endgroup$ – ron May 10 '15 at 18:41
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    $\begingroup$ So, we use the 'central' atom of the substituent group as the reference point, and connect it to the parent group (to obtain ethyl), then 'add up' any remaining carbon bonds (in this case, only one - methyl), to properly name the substituent groups. I applied this method and successfully identified (2-methylpropyl). Thank you! $\endgroup$ – imaginov May 10 '15 at 19:13
  • $\begingroup$ @imaginov 2-methylpropyl is a different structure, this is 2-methylpropan-2-yl, however tert-butyl is the preferred name. $\endgroup$ – mykhal Nov 19 '18 at 7:16

If an organic compound has dimethyl in its name than that means that it has 2 methyl groups. If they are on the same carbon they form a V in the bond line structure. An ethyl group on the other hand forms a 2 carbon unbranched zigzag in the bond line structure instead of the V that a dimethyl group forms in the bond line structure.

For the ethyl group it is standard on all bond line structures to represent it as a 2 carbon unbranched zigzag. For the dimethyl group it is standard to represent it as a V in all bond line structures.

As to how we know this it is because an ethyl group coming off of a carbon looks like propane if you just look at that carbon and the ethyl group. In fact some people in the early years of organic chemistry might have called propane ethylmethane before the prop prefix was used for alkanes. However if this was the case some of them would call propane dimethylmethane. This was because they were looking at different carbons as the central atom. If you look from one of the ends it looks like an ethyl group sticking off of methane. If you look at the center it looks like a dimethyl group sticking off of methane.

  • $\begingroup$ Thank you for clarifying. But I don't understand why this is the case - is this standard for all bond-line structures? How do we know this? $\endgroup$ – imaginov May 10 '15 at 19:22

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