Alkanes

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IntroductionContain only carbon and hydrogen atoms.

Alkanes are not considered a functional groupas they are relatively unreactive and do not undergo many chemical reactions.

All bonds between carbon atoms and between carbon and hydrogen atoms in alkanes are single bonds. *

Liquid petroleum, also called crude oil, is the main source of alkanes

The primary use of alkanes is as a fuel for heating, generating electricity, and transportation (internal combustion engines).

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Acyclic and Cyclic AlkanesMost alkanes are acyclic (noncyclic or not in a ring). Acyclic materials are classified into two subgroups:

straight or linear chains and branched chains*

Branched chain alkanes have side groups.

groups are called by various names: substituents, side chains,pendent groups,or just branches.

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Alkanes may also be cyclic; that is, the carbon atoms are connected to form a ring.

Alkanes are occasionally called aliphatic compounds or paraffins

Alkanes come in all shapes and sizes*

Shapes of alkanes*

NomenclatureAlkanes containing5 to 19 carbon atoms have Greek prefixesthey all end in ane

Formulas:The general formula for an acyclic alkane is CnH2n+2.*

Each successive molecular formula contains one additional carbon atom and two additional hydrogen atoms, a CH2 unit.

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Formulas of alkanesThis list is known as a homologous series and the individual molecules are called homologs. *

StructuresAlkanes are called saturated compounds because they contain the maximum number of hydrogen atoms that can be attached to the carbon atoms in a given compound.*

Constitutional IsomersConstitutional isomers (also) called structural isomers) are non-identical compounds with the same molecular formula.

They have the same number and kind of atoms but the atoms of each isomer are connected in a different order.

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The atoms in methane, CH4, ethane, CH3CH3, and propane, CH3CH2CH3, can only be connected one way. Each has only one constitutional isomer. Butane,C4H10, can exist as two constitutional isomers.

Different representations of constitutional isomers of C4H10.*

The two isomers are two different compounds, with different chemical and physical properties.

As the number of carbon atoms in an alkane increases, the number of constitutional isomers increasesexponentially

If the methyl groups are on opposite sides of the ring. This is the trans form.

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CIS AND TRANS ISOMERS

Cis and trans isomers are different compounds.

If the methyl groups are on the same side of the molecule (the groups are said to be cis to each other.

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Cis and trans isomers*

Alkyl GroupsRemoving the hydrogen atom from an alkane gives an alkyl group

The substituent derived from methane (CH4) by removing one hydrogen atom is a methyl group.

Similarly, removing one hydrogen atom from either carbon atom in ethane, CH3CH3, would give an ethyl group, CH3CH2 .*

For propane removing a hydrogen atom from either end of the molecule would give an n-propyl group (usually just called a propyl group).

Removing a hydrogen atom from the central carbon atom gives a different alkyl group, an isopropyl group.

Alkyl groups are not compounds

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Classification of carbon atomsA carbon atom is often classified by the number of other carbon atoms bonded to it. Primary (1) carbon atoms are bonded to one other carbon atom, Secondary (2) carbon atoms are bonded to two other carbon atoms, tertiary (3) carbon atoms have three other carbon atoms bonded to them. quaternary (4) carbon atoms are bonded to four other carbon atoms. Primary hydrogen atoms are bonded to a primary carbon atom, etc*

Physical PropertiesAll bonds in alkanes and cycloalkanes are C-C or C-H bonds. The C-C bond is nonpolar. The C-H bond is essentially nonpolar, Thus all bonds are nonpolar and therefore the molecules are nonpolar.Using the generalization like dissolves like, these materials are soluble in nonpolar solvents, such as carbon tetrachloride and other alkanes. They are immiscible (not soluble) in polar solvents like water.*

DENSITIESThe density of alkanes is about 0.7 g/mL while the density of water is about 1.0 g/mL at room temperature.

Since alkanes are less dense and immiscible with water, a mixture of the two materials forms two layers, with alkanes being the less dense upper layer. The most common example is oil floating on water.Alkanes are hydrophobic (water-fearing).*

INTERMOLECULAR FORCESThe physical state of a compound depends upon the attraction between molecules.

Since alkanes are nonpolar, intermolecular attraction between molecules results from London dispersion (LD) forces.*

*The total effect of LD force interactions increases as the size of the molecule increases.

The total intermolecular attraction forces of the lower molecular weight linear alkanes e.g., methane, propane, and butane, are small and these materials are gases at room temperature

Larger molecules have a greater summation of LD forces, sufficiently strong to make them liquids.

Pentane to heptadecane have sufficient LD forces to make them liquids at room temperature.

Higher molecular weight alkanes have even greater total LD forces. As a result they are solids at room temperature.

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Increasing LD force interactions results in increasing boiling and melting point temperatures as the molecular weight increases in these linear molecules

Generally, branched molecules boil and melt at lower temperatures owing to decreased LD force interactions*

QUESTIONSDraw a line-bond structure for hexane.Draw all the isomers for pentane, C5H12Draw a structure that contains a quaternary hydrogen atom*