CHAPTER 11: Unit 1. Definition: Alkanes & Cycloalkanes

Opening Essay

Hydrocarbons are the simplest organic compounds, but they have interesting physiological effects. These effects depend on the size of the hydrocarbon molecules and where on or in the body they are applied. Alkanes of low molar mass—those with from 1 to approximately 10 or so carbon atoms—are gases or light liquids that act as anesthetics. Inhaling (“sniffing”) these hydrocarbons in gasoline or aerosol propellants for their intoxicating effect is a major health problem that can lead to liver, kidney, or brain damage or to immediate death by asphyxiation by excluding oxygen.

Swallowed, liquid alkanes do little harm while in the stomach. In the lungs, however, they cause “chemical” pneumonia by dissolving fatlike molecules from cell membranes in the tiny air sacs (alveoli). The lungs become unable to expel fluids, just as in pneumonia caused by bacteria or viruses. People who swallow gasoline or other liquid alkane mixtures should not be made to vomit, as this would increase the chance of getting alkanes into the lungs. (There is no home-treatment antidote for gasoline poisoning; call a poison control center.)

Liquid alkanes with approximately 5–16 carbon atoms per molecule wash away natural skin oils and cause drying and chapping of the skin, while heavier liquid alkanes (those with approximately 17 or more carbon atoms per molecule) act as emollients (skin softeners). Such alkane mixtures as mineral oil and petroleum jelly can be applied as a protective film. Water and aqueous solutions such as urine will not dissolve such a film, which explains why petroleum jelly protects a baby’s tender skin from diaper rash.

We begin our study of organic chemistry with the alkanes, compounds containing only two elements, carbon and hydrogen, and having only single bonds. There are several other kinds of hydrocarbons, distinguished by the types of bonding between carbon atoms and by the properties that result from that bonding. Later in this chapter we will examine hydrocarbons with double bonds, with triple bonds, and with a special kind of bonding called aromaticity.

DefinitionHydrocarbons are compounds that contain only the elements of carbon and hydrogen. Hydrocarbons, binary compounds of carbon and hydrogen are of two types, saturated and unsaturated. In saturated hydrocarbons, all carbon-carbon bonds are single bonds. Unsaturated hydrocarbons have one or more carbon-carbon multiple bonds, double bonds, triple bonds or both. These group of compounds are called aliphatic compounds, There is another group of hydrocarbons where benzene ring is present, they are called aromatic compounds.Alkanes: Alkanes are saturated hydrocarbons in which the carbon atom arrangement is that of an unbranched and branched-chain. The formulas of all alkanes can be represented by the general formula CnH2n+2, where n is the number of carbon atoms present.

The structural formula of methane (CH4)

3D formula of CH4

Acyclic & cyclo alkanes: alkanes can be straight chain, branched chain or ring compounds. Straight chain and branched chain compounds are called acyclic compounds.  Both cyclic and acyclic compounds together are called aliphatic compounds.The first three members of the alkane family methane, ethane, propane have only one structure. In ethane and propane, if you follow the carbon chain from one end to other, you move across the same number of carbon atoms in both representations.

Ethane

propane

However, in butane there are two different arrangement possible.Four carbon atoms present in a row, straight chain alkane, called n-butaneThree carbon atoms ina row and one carbon bonded to the middle carbon. Branched chain alkane, called isobutane.  These two are constitutional isomers.

branching in isobutane

PRACTICE PROBLEM:

The line structure of ethyl cyclohexane is shown here.

Structural isomers have the same molecular formula, but the atoms are bonded together in a different order. Draw a structural isomer of ethylcyclohexane that also contains a 6-carbon ring.What are the two skeletal structures for the longest chain isomers of C5H12?

Cycloalkanes: A hydrocarbon that contains carbon atoms joined to form a ring is called a cyclic hydrocarbon. Cycloalkanes are saturated hydrocarbons in which at least one cyclic arrangement of carbon atoms are present. The formulas of all cycloalkanes can be represented by the general formula CnH2n where n is the number of carbon atoms present.