FINALS QUIZ 1 ORGANIC GROUPS

Which of the following is not a hydrocarbon?

• Carbon tetrachloride

• Chloroform

• Methanol

• All of the above

Answer explanation

Methanol is an alcohol. Carbon tetrachloride (CCl₄), despite the lack of C-H bonds is actually a haloalkane. It was originally a methane (CH₄) that has all of its hydrogen replaced by Chlorine. Chloroform (CHCl₃) is also a haloalkane.

Which hydrocarbon has the most saturated bonds?

• 1,2-dibromoethane

• 3-methylpropene

• 4-bromobutyne

• 2-chlorohexane

Answer explanation

2-chlorohexane is purely saturated and has the longest parent chain.

Which of the following is the most soluble in chloroform?

• 3-ethylbutane

• 2-Methylhexane

• 1,4-diethylpentane

• 1,3-dimethylpropane

Answer explanation

In solubility, the golden rule is "like dissolves like". So it is important to know both the polarity of the solvent and the solute. Chlorofom is polar while all the given compounds are purely hydrocarcons so they are nonpolar. So you have to choose the least nonpolar/ most polar amongst the choices as it would be the most soluble in a polar solvent. The 1,3-dimethylpropane has the least amount of LDF due to its short parent chain and branching. The least polar is 2-methylhexane.

Which of the following compounds is the least polar?

• Acetylene

• b. Ethylene

• Ethane

• water

Answer explanation

The absence of pi bond makes alkanes the least polar of the 3 aliphatic hydrocarbons. Pi electrons has higher electronegativity so it allows dipole to form. It is arrange as follows:

increasing polarity

Alkane<alkene<alkyne<water

Ethane<ethylene<acetylene<water

What correctly describes the relationship between branching of structure to IMF?

• Branching creates less surface area which decreases the LDF of the hydrocarbon.

• Branching creates more surface area which decreases the LDF of the hydrocarbon.

• Branching creates less molecular weight which increases the LDF of the hydrocarbon

• Branching creates more molecular weight which increases the LDF of the hydrocarbon.

Answer explanation

Branching is more associated with surface area than molecular weight. So if the compound is more branched, it decreases the surface area, so less LDF is formed between the C-H bonds and C-C bonds.

What is the common name for 2-methylbutane?

• Isobutane

• Isopentane

• Tert-butane

• Tert-pentane

Answer explanation

For this one, you can eliminate the tert alcohols from your choices since we only have 1 side chain which is a methyl group in carbon 2. Since there are a total of 5 carbons (4 from the parent chain - butane, 1 methyl side chain) and the side chain is in the second carbon, the common name is "isopentane".

Arrange the following compounds to decreasing boiling point: 2pts

• I -> III -> II -> IV

• IV -> II -> III -> I

• III -> I -> II -> IV

• I -> III -> IV -> II

• II -> IV -> III -> I

Answer explanation

In theory, alkanes should have higher BP than alkene and alkynes. However, when boiling point is measured in actual setting, alkynes has higher BP than alkane due to its 2 pi bonds which makes it very polar. Polar compounds generally has higher BP than nonpolar.

BP: polar> NP

actual BP: Alkyne>Alkane>Alkene

Evaluate the given statements: (a) Straight chain pentene is less dense than 1-pentane. (b) Larger molecular sized hydrocarbon has more IMF

• Both statements are true.

• Both statements are false.

• Only the 1st statement is true.

• Only the 2nd statement is true.

Answer explanation

Both are true. Alkanes are more dense than their corresponding alkenes and alkynes. Also, MW is directly proportional to IMF as the higher MW, the more bonds there are, more bonds = more LDF/ IMF.

Arrange the following compounds to increasing melting point: (2pts)

• I -> III -> IV -> II

• II -> IV -> III -> I

• III -> II -> I -> IV

• IV -> II -> I -> III

• III -> I -> II -> IV

Answer explanation

Melting point is linked with degree of unsaturation and the cis- and trans- configuration of the compound. Cis- configuration has less MP than the trans- configuration. Alkynes has less than alkene. Although (II) has more unsaturation, it has higher MW than (IV) so IV has the lowest MP followed by II. III has Cl so it has stronger IMF than I.

Arrange the following compounds to increasing reactivity: (2pts)

I. 2,3-ethylhexane

II. 4-chloropent-2-yne

III. 2-propylpent-1-ene

IV. 2-methylhex-1,3-diene

• I > II > IV > III

• III > IV > II > I

• I > III > II > IV

• I > II > III > IV

• IV > II > III > I

Answer explanation

I. 2,3-ethylhexane

II. 4-chloropent-2-yne

III. 2-propylpent-1-ene

IV. 2-methylhex-1,3-diene

Increasing reactivity in general: alkanes < alkene < alkynes

in the given, the lowest would be (I) since it is the only ethane. Followed by (III) that has 1 double bond. Between (III) and (IV), the alkyne is less reactive since there are two double bonds in (IV).