Question

Draw the correct structure for each compound. a. 2 -hexene b. 1 -heptyne c. 4,4 -dimethyl- 2 -hexene d. 3 -ethyl-4-methyl-2-pentene

Short answer

a. 2-hexene: Six-carbon chain with a double bond between carbons 2 and 3. b. 1-heptyne: Seven-carbon chain with a triple bond between carbons 1 and 2. c. 4,4-dimethyl-2-hexene: Six-carbon chain with a double bond between carbons 2 and 3, and two methyl groups on carbon 4. d. 3-ethyl-4-methyl-2-pentene: Five-carbon chain with a double bond between carbons 2 and 3, an ethyl group on carbon 3, and a methyl group on carbon 4.

Step-by-step solution

Drawing 2-hexene

Begin with a six-carbon chain as the base structure for hexene. Since the structure is an alkene with a double bond at carbon 2, draw a double bond between the second and third carbons. The remaining carbons will be connected by single bonds.

Drawing 1-heptyne

Create a seven-carbon chain to represent heptyne. As the compound is an alkyne with a triple bond at carbon 1, place a triple bond between the first and second carbons. The rest of the carbons should be connected with single bonds.

Drawing 4,4 -dimethyl-2-hexene

Start with a six-carbon chain as the main structure for hexene with a double bond at carbon 2. Then, add two methyl groups (CH3) at carbon 4. Make sure the double bond is correctly placed between the second and third carbon atoms.

Drawing 3-ethyl-4-methyl-2-pentene

Construct a five-carbon chain to represent pentene with a double bond at carbon 2. Add an ethyl group (CH2CH3) on carbon 3 and a methyl group (CH3) on carbon 4. Ensure the double bond is located between the second and third carbons.

Key concepts

Alkene Structural Representation

When representing alkenes, the key feature to include is their characteristic double bond between two carbon atoms. For example, in the compound 2-hexene, the 'hex' refers to the six carbon atoms forming the base hydrocarbon chain, and the 'ene' signifies the presence of a double bond. To draw its structure, one starts with a straight chain of carbon atoms (carbon skeleton) and places a double bond between the second and third carbons in the chain. It is essential to number the carbon atoms in the chain starting from the end closest to the double bond for correct nomenclature.

As shown in the exercise, the placement of double bonds determines the characteristics of the alkene and how it reacts chemically. The clear depiction of bonds is critical in organic chemistry, as each type of bond (single, double, or triple) has distinct reactivities and properties.

Alkyne Structural Representation

Alkynes are hydrocarbons with at least one triple bond between two carbon atoms, denoted by the suffix '-yne'. The position of the triple bond is indicated by a number placed before the name. For 1-heptyne, it means that the seven-carbon chain contains a triple bond right at the first carbon. The process for drawing 1-heptyne involves creating a continuous chain of seven carbon atoms then installing a triple bond between the first and second carbon atoms.

The structural representation of alkynes is crucial as the triple bond's location greatly affects the compound's chemistry. Precise representation of alkyne structures is fundamental to understanding reactions such as addition reactions where the triple bond may be broken to form new compounds.

Hydrocarbon Nomenclature

Nomenclature in organic chemistry is the systematic method of naming hydrocarbon compounds based on the number of carbon atoms, the type of bonds (single, double, or triple), and the presence of substituents (atoms or groups of atoms attached to the main chain). Length of the carbon chain is indicated with a prefix like 'hex' for six carbons or 'hept' for seven. The type of bonds—single (ane), double (ene), or triple (yne)—help specify the compound's family.

For instance, in '4,4-dimethyl-2-hexene', the name tells us there are two methyl groups (each a CH3) attached to the fourth carbon of a six-carbon chain that has a double bond starting at carbon two. It's important to list substituents in alphabetical order and to use commas and hyphens properly to separate numbers and descriptors. Students can ensure correct nomenclature by following these systematic rules to precisely describe the structure of hydrocarbon compounds.

Functional Group Identification

Identifying functional groups is a critical skill in organic chemistry, as functional groups are the reactive parts of molecules that define their chemical properties. Alkenes have a carbon-carbon double bond as their functional group, while alkynes feature a carbon-carbon triple bond. Other common functional groups include alcohols (-OH), carboxylic acids (-COOH), and amines (-NH2).

In the case of '3-ethyl-4-methyl-2-pentene', the molecule's basic identity as an alkene is determined by the presence of a double bond. Additionally, the location and identity of other functional groups, such as ethyl (C2H5) or methyl (CH3), are specified by numbers indicating their position on the main carbon chain. Understanding functional groups allows students to predict the behavior and reactivity of different organic molecules and apply this knowledge to synthesize new compounds or understand biological mechanisms.