Chapter 11- Alkanes

Homework 11.12, 11.14, 11.15, 11.19, 11.22, 11.28, 11.29, 11.35, 11.43, 11.49, 11.51,11.53, 11.67

Alkanes

• Alkanes are the simplest type of organic compound

• They are members of a larger class called hydrocarbons

• Hydrocarbons- a compound composed of only carbon and hydrogen

Alkanes Alkenes Alkynes Arenes

Hydrocarbons

• Alkanes are saturated hydrocarbons, meaning they only contain carbon-carbon single bonds.

• A hydrocarbon that contain carbon-carbon double bonds, triple bonds or benzene rings is said to be unsaturated.

• Structure of Alkanes:

• Each carbon has four bonds with bond angles of 109.5o, and is tetrahedral in shape.

• As we add carbons, the structure gets more complex, but each carbon is still tetrahedral.

Naming Alkanes

Group Nomenclature

Short hand

• We can condense the condensed formula further by putting like groups together in parenthesis with a subscript.

Constitutional Isomers• Constitutional Isomers- compounds that

have the same molecular formula but different structural formulas.

• By different structural formulas, we mean that they can either have different types of bonds and/or differ in their connectivity.

• For CH4, C2H6, and C3H8, there are no constitutional isomers– No variations!!

• For C4H10, there are two:

• These are different compounds and have different physical and chemical properties!!

• All compounds that have the same molecular formula but different structural formulas are constitutional isomers.

Examples• 2 alcohols C3H8O

• 2 1o amines C3H9N

• 2 aldehydes C4H8O

• 2 carboxylic acids C4H8O2

• The ability of carbon atoms to form strong, stable bonds with other carbon atoms results in a staggering number of constitutional isomers.

CH4- 1 C15H32- 4,347

C5H12- 3 C25H52- 36,797,588

C10H22- 75 C30H62- 4,111,846,763

Nomenclature of Alkanes

• Ideally, every organic compound should have a name from which its structural formula can be drawn.

• The rules are established by the International Union of Pure and Applied Chemistry.

• They are called IUPAC names for short.

Unbranched Alkanes

• IUPAC names for unbranched chains of carbon consist of two parts:– 1) Prefix- shows the number of carbons in the

chain– 2) Suffix- -ane, to show that the compound is

an alkane

Prefixes-

Branched Alkanes

• The IUPAC name for alkanes with branched chains consist of a parent name, which shows the longest chain of carbons, and substituent names that indicate the groups attached to the parent chain.

• Alkyl Group- a substituent group derived from an alkane. It is named by dropping the –ane and adding –yl.

Examples of Alkyl groups

Rules of the IUPAC system1)The name for an unbranched chain consists of a

prefix showing the number of carbons and the ending –ane.

2) For branched chains, find the longest continuous chain as the parent chain, and its name is the root name.

3) Give each substituent on the parent chain a name and number which corresponds to the carbon atom the substituent is bonded to. Use a hyphen to separate numbers and words.

4) If there is only one substituent, number the parent chain from the end that gives the substituent the lowest number.

example:

5) If the same substituent occurs more than once, number the parent chain to give the lowest set of numbers and use di, tri, tetra, penta, hexa, etc. to indicate the number of times the substituent appears. Separate numbers with commas.

Examples

6) If there are more than one type of substituent, list them alphabetically and number the parent chain to obtain the lowest set of numbers. If there are different substituents in equal positions, give the one that come first alphabetically the lowest number.

examples:

7) Do not include the numerical prefixes di, tri, tetra, etc. in alphabetizing. You also do not include sec- or tert- in alphabetizing.

examples.

SKIP

• We are skipping section B, common names, on page 330.

Sources of Alkanes• Major sources: Natural Gas and Petroleum

• Natural gas consists of about 90-95% methane, 5-10% ethane, and some propane, butane, and 2-methylpropane

• Petroleum is a thick, viscous liquid mixture of 1000’s of compounds, most of them hydrocarbons, formed from the decomposition of marine plants and animals

Sources of Alkanes

• The fundamental separation process in refining petroleum is fractional distillation

• Practically all crude petroleum that enters a refinery goes to distillation units, where it is heated to temperatures as high as 370 to 425oC and separated into fractions

• Each fraction contains a mixture of hydrocarbons that boil within a particular range.

Cycloalkanes

• A hydrocarbon that contains carbon atoms joined to form a ring are called cyclic hydrocarbons.

• When all carbons of the ring are saturated, the hydrocarbon is called a cycloalkane.– Examples:

Naming

• To name a cycloalkane, use the prefix cyclo and the parent name that corresponds to the number of carbons in the ring.

• Then name each substituent on the ring, if there are any.

• If there is only one substitituent, there is no need to give the locate or position number.

• If there are two substituents, number the ring beginning with the substituent that comes first alphabetically.– Examples:

Shapes of Alkanes and Cycloalkanes• Organic Molecules are 3-D• Alkanes

– Free rotation about each C-C bond

• Conformation- Any 3-D arrangement of atoms in a molecule that results from rotation about a single bond

• There are an infinite number of conformations due to this rotation

• Molecules will spend most of there time in the lowest energy conformations

Shapes of Alkanes and Cycloalkanes

• Cycloalkanes– Rotation is restricted due to ring structure

• Cyclopentane- most staple conformation is the envelope conformation– 4 carbons in a plane and 1 carbon that is bent

out of the plane.– All bond angles are 109.5o

Shapes of Alkanes and Cycloalkanes

• Cyclohexane- most stable in the chair conformation, all bond angles 109.5o

• The 12 C-H bonds are arranged in two different orientations

• 6 are Axial bonds, the other 6 are Equatorial bonds.

• Axial bonds extend straight up or down• Equatorial bonds extend out from the

parameter of the ring

Cis/Trans Isomerism

• Cycloalkanes with substituents on two or more carbons of the ring show a type of isomerism

• Cis/trans isomers have:– Same Molecular formula– Same order of attachment– Different arrangement of their atoms in space

due to the restricted rotation around the C-C single bonds of the ring

Cis/Trans Isomerism• The prefix cis- indicates that the substituents

are on the same side of the ring• The prefix trans- indicates that the substituents

are on the opposite side of the ring.• Cis/trans isomers differ in the orientation of

their atoms in space, so they are called stereoisomers

• Cis/trans are only one form of stereoisomers

Physical Properties of Alkanes

• Most important is the almost complete lack of polarity in cycloalkanes and alkanes

• Melting/Boiling Points– In general, both increase as you increase MW

• 1-4 carbons………gases at RT• 5-17 carbons…….liquids at RT• 18+ carbons……..white, waxy solids

• As branching increases, the surface area decreases, so London Forces are weaker, which lead to lower MP/BP

Physical Properties of Alkanes

• Solubility– Alkanes are not soluble in water– They are soluble in each other

• Reactions of Alkanes– The most important chemical property of

alkanes is their inertness– They are quite unreactive toward any of the

normal ionic reaction conditions.

Reactions of Alkanes1) Combustion

– Oxidation of hydrocarbons is the basis for their uses as energy sources for heat and power

CH4 + 2O2 CO2 + 2H2O +212 kcal/mol

CH3CH2CH3+5O2 3CO2+4H2O +530 kcal/mol

Reactions of Alkanes2) Reactions with Halogens: Halogenations

CH4 + Cl2 heat or light CH3Cl + HCl

In IUPAC naming, the name for haloalkanes comes from naming the halogen atom as a substituent and alphabetizing it along with other substituents.

F= Fluoro, Cl= Chloro, Br = Bromo, I= Iodo