Question

Which two pairs of molecules in Figure 4.5 are isomers? For each pair, identify the type of isomer.

Short answer

The forms C₄H₁₀ in (b) and butenes in (c) (forms of C₄H₈) are isomers.

They are structural isomers of chain and structural isomers of position, respectively.

Step-by-step solution

Introduction to isomers

The organic structures formed by carbon, hydrogen, and oxygen are called hydrocarbons. Examples of hydrocarbons include alkanes, alkene, alkynes, aldehydes, carboxylic acids, urea, etc. Carbon dioxide is not a hydrocarbon as it does not contain a hydrogen atom.

Isomers are molecules with similar molecular formulas but different structural formulas. Isomers can have some differences in their functions too.

Types of isomers

Isomers are compounds whose molecular formulas resemble each other but have a different arrangement of atoms, due to which they become two separate structures. Thus, there is also a difference in their properties. The isomers are of two main types—structural isomers and cis-trans isomers.

Structural isomers are compounds with the same number of carbon atoms but different arrangements. They are furthercategorized as Chain isomerism, Position isomerism, Functional group isomerism, Metamerism, and Tautomerism.

Cis-trans isomers have the functional groups on the same side of isomers (cis isomers) or on the opposite side(trans isomers).

Structural isomers

Most of the organic molecules have a carbon skeleton.Structural isomers are compounds that differ in the covalent arrangements of their atoms.The carbon skeleton is straight in one compound but may be branched-chain in another compound. The number of chances of forming isomers increases as carbon skeletons increase in size.

Fig 4.5 (b) shows isomers having the molecular formula C₄H₁₀.

Here one compound has four carbon atoms in the parent chain, and Hydrogen atoms share the free electrons. In contrast, the other has three carbon atoms with a branch at C2.

In fig 4.5 (c), the first is a butene with double bonds at C1 while the second is a butene with double bonds at C2.

Types of Isomers

The first pair in figure 4.5 (b) is a pair of chain isomers as they differ in the parent carbon chain. The second pair in figure 4.5 (c) is a pair of position isomers because the difference in position of the double bond in each figure makes them structural isomers.