The molecular formula follows the rule regarding the molecular formula of alkenes: . Therefore,  is the molecular formula for hexene.

By definition, alcohols are compounds that contain an  (hydroxyl) group bonded covalently to the end of a hydrocarbon.

Ethanol (or ethyl alcohol) is characterized as a primary alcohol since there is only one carbon-containing group that is directly attached to the carbon containing the  group.

The unsaturation number is equal to the number of rings and pi bonds in a molecule.  Here, there are 4 rings and 1 double bond, so the unsaturation number is 5.  No formula is needed to calculate unsaturation number when a structural formula is given.

Grignard reagents are very strong bases, and therefore can be spoiled by protons. Grignard reagents are so basic in fact that any protic solvent will render them useless. Dimethyl ether is the only non-protic solvent, and is therefore the correct answer. 

The molecule pictured above is known as an ether because it contains an oxygen atom within the sequence of a carbon chain. (An oxygen atom bonded to two carbons in a carbon chain).

Therefore the correct answer is ether. All other answer choices are carbonyls, meaning that they contain a carbon atom double bonded to an oxygen atom. 

IV>III>I>II

The most acidic compound is option IV because it contains a carboxylic acid group which is the most acidic organic functional group. Phenols are weak acids. Alcohols in solution are slightly less acidic than water and therefore are slightly basic. 

The molecule's longest carbon chain has 5 carbons (thus, "pent"), and the one double bond makes it an alkENE (thus "pentene"). The longest chain is a ring structure (thus "cyclopentene"). Because the IUPAC rules automatically assign the location of the first double bond to carbons 1 and 2, there is no need for a number locand.

The molecule's longest carbon chain has 4 carbons (thus, "but"), and the presence of two double bonds makes it an alkENE, more specifically, a diene (thus "butadiene"). Because there is more than one way in which the double bonds can be arranged (between C1-C2 and C2-C3, or between C1-C2 and C3-C4), it's important to place locants indicating the lower-numbered carbon in each double bond.

The molecule's longest carbon chain has 6 carbons (thus, "hex-"), and the lack of double bonds makes it an alkane (thus "hexane"). The longest chain is a ring structure (thus "cyclohexane"), and the one branching group is a carbon chain consisting of one carbon and no double bonds (a "methyl" group). Because there are no other functional groups on the molecule there is no need to put a number before the location of the methyl group (thus "methylcyclohexane").