Question

Provide correct structures for each compound. (a) 2-hexene (b) 3-heptyne (c) 3 -methyl-1-pentyne (d) 4,4 -dimethyl-2-hexene

Short answer

The compound structures are: (a) CH3-CH=CH-CH2-CH2-CH3, (b) CH3-CH2-C≡C-CH2-CH2-CH3, (c) CH3-C≡C-CH(CH3)-CH2-CH3, (d) CH3-CH=CH-C(CH3)2-CH2-CH3.

Step-by-step solution

Draw 2-hexene

Start by drawing a six-carbon chain to represent hexene. Add a double bond between the second and third carbon atoms, since the numbering indicates the double bond starts at carbon 2. Ensure that the rest of the carbon atoms have four bonds by adding hydrogen atoms.

Draw 3-heptyne

Begin with a seven-carbon chain to represent heptyne. Include a triple bond between the third and fourth carbon atoms, as indicated by the '3-' in the name. Complete the rest of the carbon atoms with hydrogen atoms to satisfy the tetravalency of carbon.

Draw 3-methyl-1-pentyne

Sketch a five-carbon chain for pentyne with a triple bond between the first and second carbons. Add a methyl group (CH3) to the third carbon of the chain. Fill in the remaining bonds of each carbon with hydrogen atoms.

Draw 4,4-dimethyl-2-hexene

First, draw a six-carbon chain for hexene with a double bond between the second and third carbons. Then, add one methyl group to the fourth carbon on each side, giving you two methyl groups on the same carbon. Ensure each carbon has four bonds by adding hydrogen atoms.

Key concepts

Structural Formulas

Understanding the structural formulas of organic compounds is pivotal for students in grasping organic chemistry. A structural formula represents the arrangement of atoms within a molecule and provides explicit details about the molecular structure. Unlike simple molecular formulas, which only give the number of each type of atom, structural formulas illustrate how those atoms are connected.

Visualizing compounds' structures as presented in an exercise is an excellent way to start. For instance, when asked to draw 2-hexene, you need to envision the six carbon atoms in a row (as 'hex' suggests hexagon for six) and then introduce a double bond starting at the second carbon. The number before the name, such as the '2-' in 2-hexene, dictates where a bond begins. Structural formulas, hence, play a pivotal role in organic chemistry as they allow chemists to showcase the specific connectivity between atoms that can greatly influence the properties and reactivity of a molecule.

Alkenes and Alkynes

Alkenes and alkynes form the primary classes of hydrocarbons characterized by carbon-carbon double and triple bonds, respectively. Understanding how to identify and name these compounds is fundamental in organic chemistry nomenclature.

Alkenes, with their C=C double bond, provide the basis to name compounds such as 2-hexene and 4,4-dimethyl-2-hexene. 'Hexene' signifies a six-carbon hydrocarbon chain with at least one double bond. Similarly, alkynes have a triple bond, evident within names like 3-heptyne and 3-methyl-1-pentyne, with 'heptyne' and 'pentyne' indicating seven and five carbon chains respectively, and at least one triple bond.

The position of the double or triple bond is crucial and is indicated by the number in front of the name. In 2-hexene, the double bond starts at the second carbon, whereas in 4,4-dimethyl-2-hexene, the presence of '4,4-dimethyl' indicates two methyl groups attached to the fourth carbon of the chain.

Hydrocarbon Chain Notation

The backbone of organic chemistry is the hydrocarbon chain, and being adept with its notation is a core skill. The notation, a combination of numbers, letters, and dashes, succinctly describes the backbone of an organic molecule and any functional groups present.

Consider '3-methyl-1-pentyne'. The number '1-pentyne' indicates that a triple bond starts at the first carbon in a pentane chain, and '3-methyl' points to a methyl group attached to the third carbon. Each part of the notation offers crucial information about the molecule's structure, guiding the reader on how to draw it out.

It's essential not only to recognize but also to be able to correctly create this chain notation. The numbers generally represent the position of bonds or substituents along the chain, while 'meth-', 'eth-', 'prop-', '-ane', '-ene', and '-yne' reveal the number of carbons and the types of bonds present. Mastery of this hydrocarbon chain notation is a stepping stone to developing a deeper understanding of organic chemistry.