Reactions Involving Ethers SAMPLE problem

48 Chapter 1 NEL

Section 1.5

Questions

1. Write structural formulas and IUPAC names for all saturated alcohols with five carbon atoms and one hydroxyl group. 2. Explain why the propane that is used as fuel in a barbecue

is a gas at room temperature, but 2-propanol used as rub- bing alcohol is a liquid at room temperature.

3. Draw the structures and write the IUPAC names of the two alkenes that are formed when 2-hexanol undergoes a con- densation reaction in the presence of an acid catalyst. 4. Write an equation using structural diagrams to show the

production of each of the following alcohols from an appro- priate alkene:

(a) 2-butanol (b) 2-methyl-2-propanol 5. Draw the structures and write the IUPAC names of all the

ethers that are isomers of 2-butanol.

6. Classify and write structural formula equations for the fol- lowing organic reactions.

(a) ethene  water→ethanol

(b) 2-butanol →1-butene  2-butene  water (c) ethoxyethane  oxygen →

(d) ethene  hypochlorous acid (HOCl_(aq)) →

2-chloroethanol (e) methanol  oxygen →

7. For each of the following pairs of compounds, select the one that has the higher boiling point. Give reasons for your answer. (a) ethylene glycol or glycerol

(b) water or methoxymethane (c) methanol or propanol (d) methoxyethane or propanol

Applying Inquiry Skills

8. Diethylene glycol, although toxic, has occasionally, and illegally, been added to wines to enhance the sweet flavour. Propose an experimental design, including the procedure and the equipment that you would use, to remove the diethylene glycol and purify the contaminated wine. Include in your answer any safety precautions needed.

Making Connections

9. Alcohols have gained increased popularity as an additive to gasoline, as a fuel for automobiles. “Gasohols” may contain up to 10% methanol and ethanol, and are considered more environmentally friendly than gasoline alone.

(a) Write balanced chemical reactions for the complete combustion of methanol and ethanol.

(b) Although methanol is less expensive to produce, ethanol is blended with methanol in gasoline. This is because methanol does not mix well with gasoline, and ethanol is used as a co-solvent. Explain, with reference to molecular structure, why ethanol is more soluble than methanol in gasoline (which is mostly octane). (c) Methanol and ethanol are considered to be more envi-

ronmentally friendly than gasoline. Research, and explain why.

(d) When small amounts of water are present in the gaso- line in the gas lines of a car, the water may freeze and block gasoline flow. Explain how using a gasohol would affect this problem.

Practice

Understanding Concepts

12. Like many organic compounds, alcohols and ethers undergo complete combustion reactions to produce carbon dioxide and water. Select one alcohol and one ether, and write stuctural diagrams for their complete combustion.

13. The major disadvantages of using ethoxyethane as an anesthetic are its irritating effects on the respiratory system and the occurrence of post-anesthetic nausea and vomiting. For this reason, it has been largely replaced by methoxypropane, which is relatively free of side effects.

(a) Draw structural formulas of ethoxyethane and methoxypropane, and determine if they are isomers.

(b) Write an equation to show the formation of ethoxyethane from ethanol.

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Functional group: R–O–R Preparation:

• alcohols → ethers  water

Condensation reaction, eliminating H₂O; dehydration ROH  ROH → ROR  HOH

Ethers

1.6

1.6

A key to the survival of a species is the ability of its members to communicate with each other, often over great distances. Insects use chemical signals to share information about location of food or of danger, and to attract potential mates. You may have seen a line of ants following a chemical trail left by other ants who have found food or water. Many of the molecules used belong to a group of organic compounds called ketones. For example, some ants use a simple ketone called 2-heptanone (Figure 1(a)) to warn of danger. A group of more complex ketones, called pheromones, are used by many insects as sex attracters. One example, 9-ketodecenoic acid (Figure 1(b)), is used by a honeybee queen when she takes flight to establish a new hive.

These pheromones are very specific to the species and are powerful in their effect. Foresters make use of the potency of pheromones to attract and trap pest insects such as the gypsy moth (Figure 2). The trapping programs are designed to control specific insect populations by inhibiting breeding—an environmentally benign alternative to spraying nonspecific insecticides.

Closely related to the ketone family is the aldehydefamily of organic compounds. These compounds also seem to be detectable over long distances, by our sense of smell. The smaller aldehydes have strong, unpleasant odours. Formaldehyde is the simplest aldehyde and is a colourless gas at room temperature; in aqueous solution, it is used as an antiseptic and a disinfectant. The next simplest aldehyde is acetaldehyde, which is a colourless liquid used in the synthesis of resins and dyes, and also as a preservative. Both formaldehyde and acetaldehyde can form solid trimers; that is, three molecules can join together into a single large molecule. The trimer of formaldehyde is used to fumigate rooms against pests, and the trimer of acetaldehyde is used as a hypnotic drug.

Aldehydes of higher molecular weight have pleasant flowery odours and are often found in the essential oils of plants. These oils are used for their fragrance in perfumes and aromatherapy products. The oil of bitter almond, for example, is benzaldehyde, the simplest aldehyde of benzene.

The functional group in ketones is the carbonyl group, consisting of a carbon atom joined with a double covalent bond to an oxygen atom. In ketones, the carbonyl group is attached to two alkyl groups and no H atoms. In aldehydes, the carbonyl group is attached to at least one H atom; it is also attached to either another H atom or an alkyl group. In other words, in an aldehyde, the carbonyl group always occurs at the end of a carbon chain; in a ketone, the carbonyl group occurs in the interior of a carbon chain.

ketone an organic compound char- acterized by the presence of a car- bonyl group bonded to two carbon atoms

aldehyde an organic compound characterized by a terminal carbonyl functional group; that is, a carbonyl group bonded to at least one H atom

H H C C C O H C H H C C H H H C H H H H H H Figure 1 (a) H H C C C O H C H H C C H H H C H H H H C C C H H C OH O H (b) Figure 2

Many ketones are volatile com- pounds that can travel long dis- tances, making them good carriers of chemical signals for insects. Ants, bees, and moths like the gypsy moths shown here, produce and detect minute quantities of ketones to communicate the presence of food or water or the availability of a mate. carbonyl group a functional group containing a carbon atom joined with a double covalent bond to an oxygen atom; CO

2-heptanone

50 Chapter 1 NEL

This difference in the position of the carbonyl group affects the chemical reactivity of the molecule, and is used in a test to distinguish aldehydes from ketones (Figure 3).

(a) carbonyl group (b) an aldehyde (c) two ketones

Later in this chapter, we will look at compounds in which the carbonyl group is attached to other groups containing oxygen or nitrogen atoms.