3
$\begingroup$

Saturated hydrocarbons: Each carbon atom is bonded to as many hydrogen atoms as possible. In other words, the carbon atoms are saturated with hydrogen (Source).

or

Saturated means that the hydrocarbon has only single bonds and that the hydrocarbon contains the maximum number of hydrogen atoms for each carbon atom Latech.edu.

But if a carbon chain has side chains, the carbon from the side chain takes the place of a hydrogen. So it doesn't have as many hydrogens as it could have right?

But then I guess I could say that anything except methane has less than the max (4) number of hydrogens.

So what exactly is meant by maximum number of hydrogens?

$\endgroup$
  • 4
    $\begingroup$ "Maximum number of hydrogens" is a poor definition, for it is ambiguous, just as you pointed out. Better stick with "no multiple bonds". $\endgroup$ – Ivan Neretin Oct 21 '16 at 19:16
  • 4
    $\begingroup$ All saturated (and non-cyclic) hydrocarbons have the general formula $\ce{ C_nH_{2n+2} }$ $\endgroup$ – Karl Oct 21 '16 at 19:26
  • $\begingroup$ @Karl You should just make that an answer... $\endgroup$ – Zhe Oct 21 '16 at 19:52
  • $\begingroup$ So is something like methanol saturated compound (not hydrocarbon). What I mean is: if a compound has all single bonds but the carbon is attached to 4 groups other than hydrogen, is it still saturated; or is the term saturated specific to hydrogen? $\endgroup$ – K-Feldspar Oct 22 '16 at 0:10
6
$\begingroup$

Standard organic chemists’ understanding is that saturated/unsaturated is a word pair entirely independent of branched/linear/cyclic. Thus, all six combinations are possible:

  • branched/saturated (e.g. neopentane or 2,2-dimethylpropane)

  • branched/unsaturated (e.g. 2,3-dimethylbut-2-ene)

  • linear/saturated (e.g. n-pentane)

  • linear/unsaturated (e.g. hex-3-ene)

  • cyclic/saturated (e.g. cyclohexane)

  • cyclic/unsaturated (e.g. cyclohexene)

I chose these examples for a reason. The central carbon atoms in both neopentane and 2,3-dimethylbut-2-ene are only bonded to other carbon atoms while all other carbons in the branched examples are methyl groups ($\ce{CH3}$). Yet, no organic chemist will classify neopentane as ‘unsaturated’ — so clearly there must be an additional criterion other than ‘maximise the number of bonds to hydrogen atoms’.

On the other hand, hex-3-ene has almost maxed out its number of hydrogen atoms: two carbons with three, two carbons with two and only two carbons which are lacking a hydrogen each. But that does not make it any less unsaturated. Note that cyclohexane and hex-3-ene have the same formula $\ce{C6H12}$ and thus the same carbon/hydrogen ratio. Yet, the cyclic compound is considered saturated and the linear isn’t.

Thus, forget about the maximum hydrogen definition. A saturated molecule is one that is made up of single bonds only.

Note: Of course, cyclic compounds can also be branched or ‘substituted’. But that doesn’t add too much.

$\endgroup$
5
$\begingroup$

It is better to stick with the definition that describes a saturated hydrocarbon as one that does not contain any double bonds, and an unsaturated one as one that does. Using this definition will allow you to clearly see whether a hydrocarbon is saturated or not. The other one is better used in order to understand what something 'saturated' is, and how something can go from being unsaturated to saturated, for example by addition of hydrogen.

$\endgroup$
3
$\begingroup$

The only hydrocarbon that has "as many hydrogen atoms as possible" is methane, with four H per C. For all other hydrocarbons, a C replaces at least one H. Whether in a straight or a branched chain, in a non-cyclic saturated hydrocarbon or alkane the number of hydrogen atoms is given in the formula @Karl cited, $\ce{C_nH_{2n+2}}$. For cycloalkanes such as cyclopropane, the formula is $\ce{C_nH_{2n}}$, since two end bonds go to carbons.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.