1. Draw 3-ethyl-2,4-dimethyloctane.
When starting with this problem we can break it into 3 parts.
The octane means there are 8 carbons in a line, forming our main chain of the hydrocarbon. The ethyl and dimethyl groups refer to the branching carbon and hydrogen groups on the main stem with the 3,4 and 2 referencing their placements on the chain.
First step is to draw out the octane chain. This will have 8 Carbon molecules connected by single bonds to each other. Next, place one ethyl molecule (CH2) connected to the third carbon in the chain (thus the 3-ethyl). After attaching the ethyl group there are still unused electrons that need to bond on the ethyl carbon and will attract another methyl group the will connect by a single bond to the ethyl group's carbon.
Finally, place a methyl group (CH3) on both the second and fourth carbons in the chain (2,4-dimethyl). Di means 2, thus we know there are 2 methyl groups. Fill in the rest of the spaces with hydrogens, giving you:
2. Naming the compound.
Our first step is to decide how many carbons are on the main chain. In this case there are 5 making it a pentane hydrocarbon. Next we find and other groups that may be attached to it, in this case there are 3. The first is found on the first carbon and will be represented as a 1-methyl in the name. The other 2 are also methyls and are found on the 4th carbon and will be called 4-dimethyl in the name. Put this all together and you get:
3. Sketch two different structural isomers of 5-carbon hydrocarbon.
A 5-carbon hydrocarbon refers to a molecule that contains 5 carbons and is entirely made of carbons and hydrogens. Structural isomers are different variations of similar chemical structures. For instance, both 2-ethylhexane and 4-methylheptane are 8-carbon hydrocarbons, but are structurally unique and are thus different molecules with different properties such as boiling points. To make a 5-carbon hydrocarbon we have a few main options for structural isomers.
1) Straight chain, CH3-CH2-CH2-CH2-CH3
2) Butane chain (4 carbons) with a methyl group, CH3-CH2-CH2-CH-CH3
3) A cross-shaped propane with 3 methyls in a line and 2 methyls bonded to the center ethyl group (2-dimethylpropane).
4) A propane chain with an ethyl group connected at the center (2-ethylpropane).
There aren't too many more options for a simple 5-carbon molecule but any of these should do. Hope this all helped!