[tiennhan]

1. Unsaturated hydrocarbons contain either double or triple bonds. How am I suppose to identify this from formulas through, other than drawing it out; is there a numerical formula to the subscripts?

1) CH2CHCl (3) CH3CH2CH3 (2) CH3CH2Cl (4) CH3CHCH2

[Yggdrasil]

Drawing the structures out is always a good idea when trying to determine the number of double or triple bonds, however there is a formula to calculate the degree of unsaturation of a compound.  The degree of unsaturation tells you the number of multiple bonds (e.g. a double bond will add one degree of unsaturation and a triple bond will add two degrees of unsaturation) or rings in a compound.

du = (1/2) (2C - H + N - X + 2)

where
C = number of carbons
H = number of hydrogens
N = number of nitrogens
X = number of halogens

and the number of oxygens present does not affect the degrees of unsaturation.  The disadvantage of this method, however, does not tell you where the multiple bonds/rings are in the molecule.

See also http://en.wikipedia.org/wiki/Degree_of_unsaturation

[tamim83]

is there a numerical formula to the subscripts

Yes.  For a saturated hydrocarbon the general molecular formula is C_nH_2n+2.  So if the molecular formula follows this then you have a saturated hydrocarbon (or an alkane I guess  )  Also, when drawing structures, carbon likes 4 bonds, nitrogen likes 3 bonds, oxygen likes 2 bonds, and halogens like 1 bond. 

[english]

1. Unsaturated hydrocarbons contain either double or triple bonds. How am I suppose to identify this from formulas through, other than drawing it out; is there a numerical formula to the subscripts?

1) CH2CHCl (3) CH3CH2CH3 (2) CH3CH2Cl (4) CH3CHCH2

You do not need numerical formulas to follow, at least for compounds this small.  Example one would be an alkene (double bonds between CH2 and CH, and a single bond between CH and Cl).  How do I know this?  For your purposes all you need to know is that carbon must have four bonds (if its formal charge is zero) to attain an octet, hydrogen must have one and only one, and chlorine must have one and only one as well. 

You don't even need Lewis structures to figure this out.  Just think, starting with the methyl group CH2-, how many bonds must it have here?  Well, we see it is bound to 2 hydrogens, so that's 4 total electrons shared.  Well that's not 8!  So we keep going.  It's also bound to the carbon of the second methyl group CH-.  OK now we have 6 shared electrons (2 electrons for each bond right!?).  This isn't at it's octet yet.  So this must mean that the two carbons of CH2 and CH are doubly bound (carbon can have four bonds remember earlier!).  So the correct way to draw this condensed structure is CH2=CHCl, where the "=" indicates a double bond.  Forget about lone pairs.  With alkane molecules all you need to do is count bonds (2 electrons each) to see if each atom has its octet.  The only time lone pairs come in is when you are dealing with alcohols (-OH) or amines (-NH2).

Hope that helps 

[Bakegaku]

Also, when drawing structures, carbon likes 4 bonds, nitrogen likes 3 bonds, oxygen likes 2 bonds, and halogens like 1 bond.

Don't forget that Hydrogen, like the halogens, also likes 1 bond.

[Donaldson Tan]

CH3CH2CH3

This compound is saturated because each carbon atom has 4 single bonds.