Chapter 21, Problem 21.4TC

Interpretation Introduction

(a)

Interpretation:

Alkene among the given hydrocarbons is to be identified.

${\text{C}}_4{\text{H}}_6$, ${\text{C}}_2{\text{H}}_6$, ${\text{C}}_{\text{7}}{\text{H}}_{12}$, ${\text{C}}_8{\text{H}}_{16}$

Concept introduction:

Hydrocarbons are the binary compounds that consist of carbon and hydrogen atoms in its structure. Alkenes are the hydrocarbons that are unsaturated and consist of at least one carbon-carbon double bond in its structure. The general representation of alkenes is ${\text{C}}_n{\text{H}}_{2n}$, $n$ is the number of carbon atoms. For examples, ethene $\left({\text{C}}_2{\text{H}}_4\right)$, and propene $\left({\text{C}}_3{\text{H}}_6\right)$.

Answer to Problem 21.4TC

Among the given hydrocarbons ${\text{C}}_8{\text{H}}_{16}$ is straight chain alkene. ${\text{C}}_4{\text{H}}_6$ and ${\text{C}}_{\text{7}}{\text{H}}_{12}$ are cycloalkenes and ${\text{C}}_2{\text{H}}_6$ is a normal alkane.

Explanation of Solution

According to the general formula of alkenes, for 8 carbon atoms, there must be 16 hydrogen atoms. Hydrocarbon, ${\text{C}}_8{\text{H}}_{16}$ satisfy the general formula for alkenes thus it is normal alkene.

According to the general formula of alkenes, for 4 carbon atoms there must be 8 hydrogen atoms, for 7 carbon atoms there must be 14 hydrogen atoms, for 2 carbon atoms, there must be 4 hydrogen atoms. Hydrocarbons ${\text{C}}_4{\text{H}}_6$ and ${\text{C}}_{\text{7}}{\text{H}}_{12}$ does not satisfy the general formula for alkenes rather they have two hydrogen atoms short. ${\text{C}}_4{\text{H}}_6$ and ${\text{C}}_{\text{7}}{\text{H}}_{12}$ are cycloalkenes and not normal alkenes. ${\text{C}}_2{\text{H}}_6$ has two hydrogen atoms extra in its structure so it is a normal alkane.

Conclusion

Among the given hydrocarbons ${\text{C}}_8{\text{H}}_{16}$ is straight chain alkene. ${\text{C}}_4{\text{H}}_6$ and ${\text{C}}_{\text{7}}{\text{H}}_{12}$ are cycloalkenes and ${\text{C}}_2{\text{H}}_6$ is a normal alkane.

Interpretation Introduction

(b)

Interpretation:

The structural formula of trans-difluoroethene $\left({\text{C}}_{\text{2}}{\text{H}}_{\text{2}}{\text{F}}_{\text{2}}\right)$ is to be determined.

Concept introduction:

Isomers are the compounds which have the same molecular formula but a different structural arrangement of atoms. Alkenes have a double bond between carbon atoms. Due to the presence of a double bond, the rotation about adjacent carbon-carbon atoms is highly restricted. Hence, alkenes can exist in two forms which have the same molecular formula but a different strucutral arrangement of atoms around the double bond. This isomerism is a type of stereoisomerism which is known as geometric isomerism or cis-trans isomerism. In alkenes, when the two similar groups are present on the same side of the double bond is known as cis-isomer and when the two same groups are on the opposite side of the double bond then is known trans-isomer.

In the IUPAC name of an alkene, the Greek prefix represents the number of the substituent, the term before the Greek prefix represents the isomer of the alkene. The suffix for alkene is ‘ene’.

Answer to Problem 21.4TC

The structural formula of trans-difluoroethene $\left({\text{C}}_{\text{2}}{\text{H}}_{\text{2}}{\text{F}}_{\text{2}}\right)$ is,

Explanation of Solution

The structural formula of trans-difluoroethene $\left({\text{C}}_{\text{2}}{\text{H}}_{\text{2}}{\text{F}}_{\text{2}}\right)$ consists of two fluorine atoms on the opposite side of the double bond. The structural formula of trans-difluoroethene $\left({\text{C}}_{\text{2}}{\text{H}}_{\text{2}}{\text{F}}_{\text{2}}\right)$ is,

Conclusion

The structural formula of trans-difluoroethene $\left({\text{C}}_{\text{2}}{\text{H}}_{\text{2}}{\text{F}}_{\text{2}}\right)$ is,

Interpretation Introduction

(c)

Interpretation:

Alkynes among the given hydrocarbons are to be identified.

${\text{C}}_4{\text{H}}_6$, ${\text{C}}_2{\text{H}}_6$, ${\text{C}}_{\text{7}}{\text{H}}_{12}$, ${\text{C}}_8{\text{H}}_{16}$.

Concept introduction:

Hydrocarbons are the binary compounds that consist of carbon and hydrogen atoms in its structure. Alkynes are the hydrocarbons that are unsaturated and consist of at least one carbon-carbon triple bond in its structure. The general representation of alkynes is ${\text{C}}_n{\text{H}}_{2n-2}$, $n$ is the number of carbon atoms. For examples, ethyne $\left({\text{C}}_2{\text{H}}_2\right)$, and propyne $\left({\text{C}}_3{\text{H}}_4\right)$.

Answer to Problem 21.4TC

Among the given hydrocarbons ${\text{C}}_4{\text{H}}_6$ and ${\text{C}}_{\text{7}}{\text{H}}_{12}$ are alkynes. ${\text{C}}_8{\text{H}}_{16}$ is straight chain alkene and ${\text{C}}_2{\text{H}}_6$ is a normal alkane.

Explanation of Solution

According to the alkynes general formula, for 4 carbon atoms there must be 6 hydrogen atoms and for 7 carbon atoms, there must be 12 hydrogen atoms. Hydrocarbons ${\text{C}}_4{\text{H}}_6$ and ${\text{C}}_{\text{7}}{\text{H}}_{12}$ satisfies the general formula for alkynes thus these are normal alkyne.

According to the alkynes general formula, for 8 carbon atoms there must be 14 hydrogen atoms and for 2 carbon atoms, there must be 4 hydrogen atoms. ${\text{C}}_8{\text{H}}_{16}$ satisfy the general formula for alkenes thus it is normal alkene and ${\text{C}}_2{\text{H}}_6$ satisfy the general formula for alkanes.

Conclusion

Among the given hydrocarbons ${\text{C}}_4{\text{H}}_6$ and ${\text{C}}_{\text{7}}{\text{H}}_{12}$ are alkynes. ${\text{C}}_8{\text{H}}_{16}$ is straight-chain alkene and ${\text{C}}_2{\text{H}}_6$ is a normal alkane.

Interpretation Introduction

(d)

Interpretation:

Straight chain isomers of pentyne and structural formula of $2\text{-pentyne}$ is to be determined.

Concept introduction:

Isomers are the compounds which have the same molecular formula but different structural formula. Alkynes are the hydrocarbons that are unsaturated and consist of at least one carbon-carbon triple bond in its structure. The general representation of alkynes is ${\text{C}}_n{\text{H}}_{2n-2}$, $n$ is the number of carbon atoms. In alkynes, the isomers differ from each other in the position of the triple bond.

Answer to Problem 21.4TC

There are two straight chain isomers of pentyne: $\text{1-pentyne}$ and $2\text{-pentyne}$. The structural formula of $2\text{-pentyne}$ is,

Explanation of Solution

There are two straight chain isomers of pentyne: $\text{1-pentyne}$ and $2\text{-pentyne}$. The difference in them is the position of the triple bond. The structure of $\text{1-pentyne}$ and $2\text{-pentyne}$ is,

Conclusion

There are two straight chain isomers of pentyne: $\text{1-pentyne}$ and $2\text{-pentyne}$. The structural formula of $2\text{-pentyne}$ is,