isomers

properties of isomers

Because isomers are different compounds, they have different properties. Generally, branched-chain isomers have lower boiling and melting points than straight-chain isomers. For example, the boiling and melting points of iso- butane are -12 C and -160 C, respectively, compared with 0 C and -138 C for n-butane. The more branching there is, the lower the boiling and melting points are. Q: The boiling point of n-pentane is 36 C. Predict the boiling points of iso-pentane and neo-pentane. A: The boiling point of iso-pentane is 28 C, and the boiling point of neo-pentane is 10 C.

how many isomers

Butane has only two isomers and pentane has just three, but some hydrocarbons have many more isomers than these. As you increase the number of carbon atoms in a hydrocarbon, the number of isomers quickly increases. For example, heptane, with seven carbon atoms, has nine isomers; and dodecane, with twelve carbon atoms, has 355 isomers. Some hydrocarbons with many more carbon atoms have billions of isomers! Q: Why does the number of carbon atoms in a hydrocarbon determine how many isomers it has? A: The more carbon atoms there are, the greater the number of possible arrangements of carbon atoms.

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examples of isomers

The smallest hydrocarbon that has isomers is butane, which has just four carbon atoms. In the Figure 1.1 you can see structural formulas for normal butane (or n-butane) and its only isomer, named iso-butane. Both molecules have four carbon atoms as well as ten hydrogen atoms (C4 H10 ), but the atoms are arranged differently in the two compounds. In n-butane, all four carbon atoms are lined up in a straight chain. In iso-butane, one of the carbon atoms branches off from the main chain. The next smallest hydrocarbon is pentane, which has five carbon atoms and twelve hydrogen atoms (C5 H12 ). Pentane has three isomers: n-pentane, iso-pentane, and neo-pentane. Their structural formulas are shown in the images below. Look at the carbon atoms in each isomer. In n-pentane (see Figure 1.2), the carbon atoms form a straight chain. In iso-pentane (see Figure 1.3), one carbon atom branches off from the main chain. In neo-pentane (see Figure 1.4), two carbon atoms branch off from the main chain.

same atoms different shapes

Hydrocarbons are compounds that contain only carbon and hydrogen atoms. The smallest hydrocarbon, methane (CH4 ), contains just one carbon atom and four hydrogen atoms. Larger hydrocarbons contain many more. Hydro- carbons with four or more carbon atoms can have different shapes. Although they have the same chemical formula, with the same numbers of carbon and hydrogen atoms, they form different compounds, called isomers. Isomers are compounds whose properties are different because their atoms are bonded together in different arrangements.

instructional diagrams

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questions

only unsaturated hydrocarbons have isomers.

a. true

-->  b. false

which statement about the isomers of a given hydrocarbon is true?

a) they have the same chemical formula.

b) they have the same structural formula.

c) they have the same number of carbon atoms.

-->  d) two of the above

how many isomers does pentane have?

a) 1

b) 2

-->  c) 3

d) 4

some hydrocarbons have billions of isomers.

-->  a. true

b. false

the more branching an isomer has, the lower its melting point is.

-->  a. true

b. false

diagram questions

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