>Professor Strauss,

>

>I am having problems with writing structural formulas for a compound when just

>the molecular formula is given. For example, problem 2.23 on page 86 asks you

>to "write structural formulas for four compounds with the formula C3H6O..." Do

>you have any tips on how to do this? Thanks.

>

>Grace Sullivan

Grace,

In the beginning it will be a trial and error process for you. I

would begin by trying three C's in a row and an O, and then putting in all

the hydrogens:

C C C O

If you put in all the H's, CH3-CH2-CH2-OH, you will see that 8 are

required if no double bonds or rings are present, so the structures you

must write must have one double bond or a ring since you must have only six

hydrogens. You can write many structures like this. Start with the

framework above and you will get:

H2C=CH-CH2-OH as the first (an alcohol)

CH3-CH=CH-OH as the second (and there are two geom isomers

of this one)

Then put the O in between the carbons and continue the process:

CH3-O-CH=CH2 (an ether)

Then write branched structures in which the O comes off the middle carbon..

CH2=CH(OH)-CH3

and so on... now continue...but put the double bond to the O

like: CH3-CH2-CH=O (an aldehyde)

or CH3COCH3 (a ketone)....

Notice that you are writing the structures of alcohols, ethers, aldehydes,

ketones, and so on...

You can continue this by beginning to write 3 and four membered

ring structures, i.e.,

these are cyclic ethers....

and so on....

This question anticipates the idea of "index of hydrogen deficiency" (see

index of hydrogen deficiency in the index of your text), which tells you

whether rings and/or double bonds, or triple bonds, and how many of each,

will be required in a structure, based on the molecular formula. You then

just "begin" working... trial and error as you go..to see what works.

THE MORE YOU PRACTICE DOING THIS, THE BETTER YOU WILL GET AT IT.....