Chapter 19, Problem 70AE

(a)

Interpretation Introduction

Interpretation:

The possible open-chain structures for the given molecular formula, ${\text{C}}_{\text{5}}{\text{H}}_{\text{8}}$ has to be drawn and the IUPAC names for the corresponding open-chain structures has to be given.

Concept Introduction:

Hydrocarbons:

Hydrocarbons contain only carbon and hydrogen atoms which are connected together by covalent bonds such as $\text{C}-\text{C}$, $\text{C=C}$, $\text{C}\equiv \text{C}$ and $\text{C}-\text{H}$. It can be categorized into two types: saturated hydrocarbon and unsaturated hydrocarbon.

Alkynes:

Alkynes are unsaturated hydrocarbon that must contain a carbon-carbon triple bond in it. The general molecular formula for open-chain alkanes is ${\text{C}}_n{\text{H}}_{2n-2}$. Here, $n$ indicates the number of carbon atoms in the compound.

The general structure of alkyne is shown here:

IUPAC rules for naming alkenes and alkynes:

• Identify the parent carbon chain in the given compound that must contain the carbon-carbon double or triple bonds. If two or more chains have the same carbon lengths, choose the chain with a greater number of substituents bonded to it.
• The alkene or alkyne name is derived from the corresponding alkane. Thus, the parent compound name is derived by replacing the $\text{-}ane$ ending of the alkane to $\text{-}ene$ for the alkene and $\text{-}yne$  for the alkyne. If two or more structural isomers are possible, use the position number for the alkene and alkyne groups.
• The parent carbon chain is always numbered from the begning of the chain only if the alkene or alkyne group has the lowest possible locant.
• Identify and name the alkyl groups including their positions on the parent carbon chain.
• If the parent carbon chain contains two or more same kinds of alkyl groups, indicate this repetition by a numerical multiplier ($di\text{-}$, $tri\text{-}$ and so on) used before the alkyl-group name. The numbers for alkyl group positions are separated by a comma and the alkyl-group name and the position number is separated by a hyphen.
• If the parent carbon chain contains several different alkyl groups, arrange them in alphabetical order (do not consider numerical multipliers). Write all alkyl group names that include their locants in front of the parent compound’s name.

Explanation of Solution

The given molecular formula for pentyne is ${\text{C}}_{\text{5}}{\text{H}}_{\text{8}}$.  The given compound is unsaturated hydrocarbon (alkyne).

There are five carbon atoms in the given compound. Place the five carbon atoms in a straight-line as the parent carbon chain as shown below.

Add the carbon-carbon triple bond between $\text{C-1}$ and $\text{C-2}$ for terminal alkyne and between $\text{C-2}$ and $\text{C-3}$ for internal alkyne. Now, add the hydrogen atoms to each carbon atom in order to complete the structure because each carbon has four bonds around it.

So, the structures for the alkyne compound is as follows:

In pentyne, there are five carbon atoms in the parent carbon chain so that the parent compound’s name is $\text{pentyne}$.

The parent carbon chain is numbered from the left end of the carbon chain as shown in the above structure. There is a carbon-carbon triple bond between $\text{C-1}$ and $\text{C-2}$. Hence, the IUPAC name for the given compound is $\text{1-pentyne}$.

Similarly,

The parent carbon chain is numbered from the left end of the carbon chain as shown in the above structure. There is a carbon-carbon triple bond between $\text{C-2}$ and $\text{C-3}$. Hence, the IUPAC name for the given compound is $\text{2-pentyne}$.

Hence, the possible open-chain structures with their IUPAC names for pentyne are shown below.

(b)

Interpretation Introduction

Interpretation:

The possible open-chain structures for the given molecular formula, ${\text{C}}_{\text{6}}{\text{H}}_{\text{10}}$ has to be drawn and the IUPAC names for the corresponding open-chain structures has to be given.

Concept Introduction:

Refer to part (a).

Explanation of Solution

The given molecular formula for hexyne is ${\text{C}}_{\text{6}}{\text{H}}_{\text{10}}$.  The given compound is unsaturated hydrocarbon (alkyne).

There are six carbon atoms in the given compound. Place the six carbon atoms in a straight-line as the parent carbon chain as shown below.

Add the carbon-carbon triple bond between $\text{C-1}$ and $\text{C-2}$ for terminal alkyne and between $\text{C-2}$ and $\text{C-3}$, and $\text{C-3}$ and $\text{C-4}$ for internal alkynes. Now, add the hydrogen atoms to each carbon atom in order to complete the structure because each carbon has four bonds around it.

So, the structures for the alkyne compound is as follows:

In hexyne, there are six carbon atoms in the parent carbon chain so that the parent compound’s name is $\text{hexyne}$.

For the first isomer:

The parent carbon chain is numbered from the left end of the carbon chain as shown in the above structure. There is a carbon-carbon triple bond between $\text{C-1}$ and $\text{C-2}$. Hence, the IUPAC name for the given compound is $\text{1-hexyne}$.

For the second isomer:

The parent carbon chain is numbered from the left end of the carbon chain as shown in the above structure. There is a carbon-carbon triple bond between $\text{C-2}$ and $\text{C-3}$. Hence, the IUPAC name for the given compound is $\text{2-hexyne}$.

For the third isomer:

There is a carbon-carbon triple bond between $\text{C-3}$ and $\text{C-4}$. Hence, the IUPAC name for the given compound is $\text{3-hexyne}$.

Hence, the possible open-chain structures with their IUPAC names for pentyne are shown below.