The structures didn't copy very well, but if you are comparing
CH3
|
CH3-CH-CH-CH2-CH2-CH3
|
CH2
|
CH3
and
CH3
|
CH3-CH2-CH2-CH-CH-CH3
|
CH2
|
CH3
then it doesn't matter, since you would arrive at the same name in each case, 3-ethyl-2-methylhexane. If you get the same name, it must the same compound!
On the other problems,
a) OK but missing a hydrogen
b) OK but the name should be 4-ethyl-3,3,4-trimethyloctane (alphabetize the groups)
c) OK (well that's an easy one!)
d) OK
e) OK but the name should be 3-ethyl-2-methylhexane
f) The name should be 3,3-diethyl-4,5-dimethyl-5-propyldecane
g) OK but missing a hydrogen
h) OK
i) Replace two hydrogens on carbons 1 and 3 with methyl groups:
H H
| |
H–C––––�?STRONG>C–H
| |
| |
H�?STRONG>C–––––C–H cyclobutane
| |
H H
You can have the cis isomer (both CH3 groups on the same side of the ring, top side or bottom side) or the trans isomer (the CH3 groups are on opposite sides of the ring).
j) Dimethylpropane. This is a case where, strictly speaking, numbers aren't needed because there is only one possible "dimethylpropane" which is 2,2-dimethylpropane. If you put a CH3 on an end carbon, it will be a butane, not a propane anymore, and if you put both CH3s on the ends, you'll have pentane! 
Steve
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