Question

Draw the carbon backbone for each molecule. a) 3 -ethyl-4-methylhept-3-ene b) 3,3 -diethylpent-1-yne

Short answer

(a) Draw a 7-carbon chain with a double bond at C3; ethyl at C3, methyl at C4. (b) Draw a 5-carbon chain with a triple bond at C1; ethyl groups at C3.

Step-by-step solution

Identify the Parent Chain

In part (a), the parent chain is 'hept-3-ene', which indicates a 7-carbon chain with a double bond at the third carbon. In part (b), the parent chain is 'pent-1-yne', which indicates a 5-carbon chain with a triple bond at the first carbon.

Draw the Parent Chain

For (a), draw a linear 7-carbon chain and indicate a double bond between the 3rd and 4th carbon. For (b), draw a linear 5-carbon chain and indicate a triple bond starting at the 1st carbon.

Identify the Substituents

In part (a), you have an ethyl group at carbon 3 and a methyl group at carbon 4. In part (b), you have diethyl groups at carbon 3.

Add the Substituents to the Parent Chain

For (a), attach the ethyl group to the 3rd carbon and the methyl group to the 4th carbon. For (b), attach one ethyl group on each side of the 3rd carbon.

Verify the Backbone Structure

Count the total number of carbons: (a) should have a total of 9 carbons (7 in the chain, plus 1 ethyl and 1 methyl), and (b) should have 7 carbons (5 in the chain, plus 2 ethyl groups). Ensure all bonds and substituent positions are correct per descriptions in previous steps.

Key concepts

Carbon Chain Drawing

When approaching organic chemistry exercises involving carbon structures, it all starts with understanding the carbon chain, also known as the 'parent chain.' This chain forms the skeleton of your molecule. For example, in the molecule name '3-ethyl-4-methylhept-3-ene,' 'hept' denotes a 7-carbon linear structure. It's essential to identify it first.
The numerical position, such as '3-ene,' tells us where the important functional parts, like double or triple bonds, are located. If you see 'hept-3-ene,' you should depict a chain with seven connected carbon atoms and a double bond between the third and fourth carbon atoms.
Ensure each carbon forms the four bonds it's capable of, including those needed to attach substituent groups. Begin by drawing a simple zigzag line when sketching these chains, as it often represents the most stable configuration.

Substituent Placement

Substituents are additional atoms or groups of atoms attached to the parent chain, modifying the molecule's basic structure and characteristics. Identifying the correct position for each substituent is crucial. Looking at '3 -ethyl-4-methylhept-3-ene,' the numbers before each substituent ('ethyl' and 'methyl') indicate where they should be placed on the main chain.

• An ethyl group means adding a side chain with two carbon atoms.
• A methyl group is a side chain with one carbon atom.

In this example, you attach the ethyl group to the third carbon and the methyl group to the fourth carbon. This gives the chain unique chemical properties, affecting how it reacts and bonds with other molecules. Double-check substituent placements against their position markers to prevent errors.

Double and Triple Bonds

The presence of double and triple bonds in a carbon chain affects both its shape and its reactivity. Recognizing and properly placing these bonds is critical in organic chemistry.

• Double bonds, indicated by '-ene,' mean that two carbons share two pairs of electrons. In our example, 'hept-3-ene' shows a double bond between the third and fourth carbon atoms.
• Triple bonds, denoted by '-yne,' involve three pairs of shared electrons. In the name 'pent-1-yne,' this triple bond starts with the first carbon atom.

Understanding these bonds helps predict how molecules interact. For instance, double and triple bonds create points in the chain with increased reactivity and can affect the three-dimensional configuration of the molecule. When drawing, carefully indicate these bonds as they significantly impact molecular behavior and further synthesis possibilities.