I have been studying alkanes, alkenes & alkynes for a while, and I see in the examples that all the basic structural formulas start with $\ce{CH3}$, then $\ce{CH2}$, and the last carbon atom is $\ce{CH3}$, but the one that is attached to the single/double/triple bond is $\ce{CH}$. (Please correct me if I'm explaining it with the right terms.)

Anyways, there's a question about drawing the structural formula for compounds.

Draw a structural formula for hex-3-ene.

What my answer is:


I thought this was right, then I looked at the solutions and it was


So now my question is, why does it start with $\ce{H3C}$ not $\ce{CH3}$ like some other examples?


2 Answers 2


Your answer is the same as the book's. The book wrote the first carbon as $\ce{H3C}$ to stress that the chain continues via a carbon to carbon bond, not bonded a carbon to hydrogen to carbon bond.

As for why it's not like this in other examples, technically it is not necessary to write it this way, but it's strange your book lacks that consistency.

  • $\begingroup$ So I'm right too? $\endgroup$ Commented Jul 20, 2015 at 1:43
  • $\begingroup$ You are indeed. $\endgroup$
    – ringo
    Commented Jul 20, 2015 at 1:43
  • $\begingroup$ Could you also write 3HC? Just curious. I write from right to left so I put index on left side. $\endgroup$
    – bodacydo
    Commented Jul 20, 2015 at 2:10
  • 4
    $\begingroup$ To my knowledge, the number is always written after the atomic symbol when denoting the number of that element. $\endgroup$
    – ringo
    Commented Jul 20, 2015 at 3:14
  • 9
    $\begingroup$ You can't use 3HC. The number of atoms has to be on the right side of the element's symbol. Otherwise, H2O could mean both 2 Hydrogens + 1 Oxygen and 1 Hydrogen + 2 Oxygen, and this lack of consistency would obviously be very problematic. Also, even though the number of atoms should be subscript, numbers to the left of a formula represent number of moles of the substances when balancing equations. $\endgroup$
    – Molx
    Commented Jul 20, 2015 at 3:28

The "H" part of H3C is to the outside of the center to stress that the carbon atom is oriented towards the other carbon atoms in the center.

  • 3
    $\begingroup$ Welcome to Chemistry.SE! Take the tour to get familiar with this site. Mathematical expressions and equations can be formatted using $\LaTeX$ syntax. For more information in general have a look at the help center. Somehow I fail to see, how this answer differs from the one that was already given. $\endgroup$ Commented Aug 4, 2015 at 4:14

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