3-methylhexane and 2,2,3-trimethylbutane

These two compounds are among the nine possible isomers of alkanes with formula C7H16:

heptane , CH3CH2CH2CH2CH2CH2CH3 , with a straight chain of seven carbon atoms.

2-methylhexane , CH3CH(CH3)CH2CH2CH2CH3 , where the longest chain consists of six carbon atoms, and a methyl group is attached to the second carbon.

3-methylhexane , CH3CH2CH(CH3)CH2CH2CH3 , where the longest chain consists of six carbon atoms, and a methyl group is attached to the third carbon.

2,3-dimethylpentane , CH3CH(CH3)CH(CH3)CH2CH3 , where the longest chain consists of five carbon atoms, and there is a methyl group attached to the second carbon and a second methyl group attached to the third carbon.

2,4-dimethylpentane , CH3CH(CH3)CH2CH(CH3)CH3 , where the longest chain consists of five carbon atoms, and methyl groups are attached to the second and to the fourth carbons.

2,2-dimethylpentane , CH3C(CH3)2CH2CH2CH3 , where the longest chain consists of five carbon atoms, and two methyl groups are attached to the second carbon.

3,3-dimethylpentane , CH3CH2C(CH3)2CH2CH3 , where the longest chain consists of five carbon atoms, and two methyl groups are attached to the third carbon.

3-ethylpentane , CH3CH2CH(C2H5)CH2CH3 , where the longest chain consists of five carbons, and there is an ethyl group attached to the third carbon.

2,2,3-trimethylbutane , CH3C(CH3)2CH(CH3)CH3 , where the longest chain consists of four carbon atoms, with two methyl groups attached to the second carbon, and one methyl group attached to the third carbon.

IS IT POSSIBLE TO HAVE A 2-ethylpentane?

No. When you draw this, you find that the longest chain has six carbons, so it is 3-methylhexane.

IS IT POSSIBLE TO HAVE A 4,4-dimethylpentane?

No. You count the carbons from the end closest to a branch, so this would be a 2,2-dimethylpentane.