• Carbon is unique among elements in that it can bond to other carbon atoms to form chains containing as many as several thousand atoms.

Millions and Millions of Organic Compounds

Organic Chemistry: Basic Concepts

Millions and Millions of Organic Compounds

• Because a carbon atom can bond to as many as four other atoms at once, these chains can have branches and form closed-ring structures that make possible an almost endless variety of compounds.

• In addition, carbon can bond strongly to elements such as oxygen and nitrogen, and it can form double and triple bonds.

Millions and Millions of Organic Compounds

Saturated Hydrocarbons

A hydrocarbon in which all the carbon atoms are connected to each other by single bonds is

called a saturated hydrocarbon. Another name for a saturated hydrocarbon is an

alkane

Alkanes• Alkanes are the simplest hydrocarbons. The

carbons in an alkane can be arranged in a chain or a ring, and both chains and rings can have branches of other carbon chains attached to them. Alkanes that have no branches are called straight-chain alkanes. Methane, CH4; ethane, C2H6; propane,

• C3H8; and butane, C4H10 are all common fuels.

Alkanes

• The carbons in an alkane can be arranged ina chain or a ring, and both chains and rings can have branches of other carbon chains attached to them.

Alkanes

• Alkanes that have no branches are called straight-chain alkanes.

• Some alkanes have a branched structure.

Alkanes

• In these compounds, a chain of one or more carbons isattached to a carbon in the longest continuous chain, which is called the parent chain.

• The carbon atoms in alkanes can also link up to form closed rings. The most common rings contain five or six carbons.

Alkanes

• The structures of these compounds can be drawn showing all carbon and hydrogen atoms.

• Structural diagrams can be simplified by using straight lines to represent the bonds between atoms in the rings.

Alkanes

• In these ring diagrams, each corner represents a carbon atom.

• Structural diagrams of straight- and branched-chain hydrocarbons also can be written in a simplified way by leaving out some of the bonds.

Alkanes

• For example, the formula for propane can be written as

CH3—CH2—CH3.

• In an even more simplified type of shorthand, the condensed structural formula for propane can be written as CH3CH2CH3.

Alkanes

• Here, the bonds both between C and C and between C and H are understood.

The First Ten Alkanes

• To name a branched alkane, you must be able to answer three questions about its structure.

Naming Alkanes

1. How many carbons are in the longest continuous chain of the molecule?

2. How many branches are on the longest chain and what is their size?3. To which carbons in the longest chain

are the branches attached?

• For convenience, the carbon atoms in organic compounds are given position numbers.

Naming Alkanes

• In straight-chain hydrocarbons, the numbering can begin at either end. It makes no difference.

.

• In branched hydrocarbons, the numbering begins at the end closest to the branch.

• Four carbons are in the longest continuous chain, so butane is the parent chain and will be part of the compound’s name.

• There is only one branch, and it contains one carbon.

• Four carbons are in the longest continuous chain, so butane is the parent chain and will be part of the compound’s name.

Naming Alkanes

• There is only one branch, and it contains one carbon.

Isomers

• Compounds that have the same formula but different structures are called isomers.

• Butane and 2-methylpropane are known as structural isomers.

• Each has the molecular formula C4H10, but they have different structural formulasbecause the carbon chains have different shapes.

Properties of Alkanes• Properties are affected by the structure or

arrangement of atoms present in a molecule.

• Another factor that affects properties of alkanes is chain length.

• In general, the more carbons present in a straight-chain alkane, the higher its melting and boiling points.

• A property shared by all alkanes is their relative unreactivity.

Properties of Alkanes• Because alkanes don’t have any polar bonds, they

undergo only a small number of reactions and will dissolve only thoseorganic compounds that are nonpolar or that have low polarity, such as oils and waxes.

Four ways of representing molecules

Unsaturated Hydrocarbons

• A hydrocarbon that has one or more double or triple bonds between carbons is called an unsaturated hydrocarbon.

• Two carbon atoms can share one, two, or three pairs of electrons. I

• An saturated hydrocarbons, carbon atoms share only one pair, whereas in unsaturated hydrocarbons, the carbon atoms participating in double or triple bonds share two or three pairs.

Unsaturated Hydrocarbons

Alkenes• A hydrocarbon in which one or more double

bonds link carbon atoms together is called an alkene.

• Alkenes are named using the root names of the alkanes, with the -ane ending changed to -ene.

• The simplest alkene is ethene, , which contains two carbons linked in a chain.

Alkenes• What is the name of the compound that has the

following structure?

• This compound has four carbons in a chain with one double bond, so butene will be part of its name.

Alkenes

• Numbering the carbons starting on the left side of the compound gives the first carbon that is part of the double bond the position number of one.

• Thus, this compound is 1-butene.

Alkenes• The formation of a double bond prevents the carbons on

each side of the bond from rotating with respect to each other.

• If the two groups attached to either carbon are different, the alkene can have two different geometric structures.

• These structures are geometric isomers.

Alkenes

• In the isomer called cis-2-butene, the hydrogen atoms and— CH3 groups are on the same side of the double bond.

Alkenes

Organic Chemistry: Basic Concepts

• In trans-2-butene, the hydrogen atoms and—CH3 groups are on opposite sides of the double bond.

Topic 24

Alkenes

• Alkenes are more reactive than alkanes because the two extra electrons in the double bond are not held as tightly to the carbons as are the electrons in a single bond.

• Alkenes readily undergo synthesis reactions in which smaller molecules or ions bond to the atoms on either side of the double bond.

Alkenes

• An unsaturated alkene can be converted into a saturated alkane by adding hydrogen to the double bond.

• This reaction is called hydrogenation.

Alkynes

• Another type of unsaturated hydrocarbon, called an alkyne, contains a triple bond between two carbon atoms.

• Alkynes are named using the alkane root name for a given carbon chain length and changing the -ane ending to -yne.

• Ethyne, known more commonly as acetylene, is the most important commercial alkyne.

• Melting and boiling points of alkynes increase with increasing chain length, just as they did for alkanes and alkenes. Alkynes have physical and chemical properties similar to those of alkenes; their melting points are higher than those of alkanes, and they undergo synthesis reactions.

Aromatic Hydrocarbons

• Another group of unsaturated hydrocarbons has distinctive, six-carbon ring structures.

• The simplest compound in this group is benzene, with the molecular formula C6H6.

• Benzene contains six carbons joined together in a flat ring.

Aromatic Hydrocarbons

• The name aromatic hydrocarbon is used to describe a compound that has a benzene ring or the type of bonding exhibited by benzene.

• Aromatic hydrocarbons were originally named because most of them have distinctive aromas.

• Although it contains double bonds, benzene does not share most of the properties of alkenes. It is unusually resistant to hydrogenation,

• Whereas most alkenes readily become hydrogenated. To account for this inertness, chemists have suggested that the extra electrons are shared equally by all six carbons in the ring rather than being located between specific carbon atoms.

Benzene

• In this hexagon, each corner represents a carbon atom.

• The circle in the middle of the structure represents the cloud of six electrons that are shared equally by the six carbon atoms in the molecule.

Sources of Organic Compound

• Most hydrocarbons come from fossil fuels, especially petroleum, but also natural gas and coal, Other important sources include wood and the fermentation products of plant materials.

• Natural gas contains large quantities of methane, along with smaller amounts of alkanes up to about five carbons in length.

• One property is boiling point. Distillation is a technique used to separate substances that have different boiling points. In the petroleum industry, huge towers are used to distill petroleum into its component liquids. Inside the tower, many plates provide multiple surfaces on which repeated vaporization-condensation cycles

take place. Repeated cycles provide for more efficient separations and allow fractions containing only one or a few different compounds to be isolated. This method of separation is called fractional distillation.

crackingThe hydrocarbons in the fractions coming off the

lower parts of the tower are mostly large alkanes. These can be converted into smaller, more useful alkanes and alkenes by a process called cracking. Cracking uses catalysts or high temperatures in the absence of air to break down or rearrange large hydrocarbons. Cracking is also used to increase the yield of natural gas by producing small alkenes from larger molecules. The cracking of propane produces some methane and ethene, in addition to propene and hydrogen.

• Another process that uses heat, pressure, and catalysts to convert large alkanes into other compounds is called reforming. It is used to form aromatic hydrocarbons.

Coal

Coal is formed from the remains of plants that became buried underwater and were subjected

• to increasing pressures as layers of mud built up. Coal is composed primarily of carbon but also contains many mineral impurities. It is used

• mostly as a fuel and as a source of aromatic hydrocarbons. Coal must be obtained from underground or surface mines,