Chapter 2

The Nature of Organic Compounds:Alkanes

Functional Groups

• The structural features that make it possible to classify compounds by reactivity are called functional groups

• A given functional group behaves almost the same way in every molecule it’s a part of

Carbon-Carbon Multiple bonds

Carbon singly bonded to an electronegative atom

Carbon-Oxygen Double Bond (carbonyl groups)

(-al) (-one) (-oic acid)

Alkanes and Alkyl Groups: Isomers

• HydrocarbonHydrocarbon: a compound composed only of carbon and hydrogen

• Saturated hydrocarbonSaturated hydrocarbon: a hydrocarbon containing only single bonds

• AlkaneAlkane: a saturated hydrocarbon whose carbons are arranged in an open chain – General molecular formula, CnH2n+2

– Homologous series, -CH2- (methylene) group

• Aliphatic hydrocarbonAliphatic hydrocarbon: another name for an alkane

Constitutional Isomerism• Constitutional isomersConstitutional isomers: compounds with the

same molecular formula but a different connectivity (order of attachment of their atoms)

• Molecular formula Constitutional isomersConstitutional isomers CH4 1

C2H6 1

C3H8 1

C4H10 2

C5H12 3

C10H22 75

C15H32 4347

• Straight-chain alkanes (normal alkanes)– n-butane

• Branched-chain alkanes– Isobutane (2-methylpropane)

Substituent group Alkyl groups (R-); suffix -yl

Classification of C & HClassification of C & H

Primary (1°) CPrimary (1°) C: a carbon bonded to one other carbon

– 1° H: a hydrogen bonded to a 1° carbon

Secondary (2°) CSecondary (2°) C: a carbon bonded to two other carbons

– 2° H: a hydrogen bonded to a 2° carbon

Tertiary (3°) CTertiary (3°) C: a carbon bonded to three other carbons

– 3° H: a hydrogen bonded to a 3° carbon

Quaternary (4°) CQuaternary (4°) C: a carbon bonded to four other carbons

Naming Branched-chain Alkanes

IUPAC system of nomenclature(International Union of Pure and Applied Chemistry)

Naming Branched-chain Alkanes

• Step1 Find the parent hydrocarbon1. Find the longest continuous carbon chain in

the molecule

2. If two chains of equal length are present, choose the one with the larger number of branch points as the parent

• Step1 Find the parent hydrocarbon

• Step2 Number the atoms in the main chain– If there is one substituent, number from the end

of the chain that gives it the lower number– If more than one substituents is attached to the

longest continuous chain, the chain number in the direction that will result in the lowest possible number

• Step1 Find the parent hydrocarbon

• Step2 Number the atoms in the main chain

• Step3 Identify and number the substituents– Assign a number to each substituent according

to its point of attachment on the parent chain– If there are two substituents on the same

carbon, assign them both the same number

Naming Branched-chain Alkanes

• Step1 Find the parent hydrocarbon• Step2 Number the atoms in the main chain• Step3 Identify and number the substituents• Step4 Write the name as a single word

– Use hyphens to separate the various prefixes and commas to separate numbers

– If two or more different side chains are present, cite them in alphabetical order

– If two or more identical side chains are present, use one of the di-, tri-, tetra-, and so forth. Don’t use these prefixes for alphabetizing

Suffix Class

-e-ol-al -one-oic acid

hydrocarbonalcoholaldehydeketonecarboxylic acid

one or more triple bondsone or more double bonds all single bonds

-yn- -en--an-

Nature of Carbon-Carbon Bonds in the Parent ChainInfix

Properties of Alkanes

Physical Properties • Low-molecular-weight alkanes

(methane....butane) are gases at room temperature

• Higher-molecular weight alkanes (pentane, decane, gasoline, kerosene) are liquids at room temperature

• High-molecular weight alkanes (paraffin wax) are semisolids or solids at room temperature

Dispersion force

Intermolecular forces of attraction• Ion bonding (188 kcal/mol)

• Hydrogen bonding (2-10 kcal/mol)

• Dispersion force (0.02-2 kcal/mol)

Physical properties• Constitutional isomers have different physical

properties

NameDensity (g/mL)

hexane2-methylpentane

3-methylpentane

2,3-dimethylbutane

2,2-dimethylbutane

68.7

60.3

63.358.0

49.7

-95

-154

-118-129

-98

0.659

0.653

0.6640.661

0.649

bp (°C)

mp (°C)

Constitutional isomers have different physical properties

bp: more branch, lowermp: branch, decrease; but symmetry, increase

• Reactions of alkanes:– The reaction of an alkane with O2 occurs during

combustion in an engine or furnace when the alkanes is used as a fuel

– Carbon dioxide and water are formed as products, and a large amount of heat is released

CH4 + 2 O2 → CO2 + 2 H2O + 890 KJ

(213 Kcal)

Conformations of Ethane

• The 3-dimentional arrangements of atoms that result from rotation around a single bond are called conformations

• They interconvert too rapidly for them to be isolated

• Staggered conformationStaggered conformation: a conformation about a carbon-carbon single bond where all six C-H bonds are as far away from one another as possible

• Eclipsed conformation: a conformation about a carbon-carbon single bond where the six C-H bonds are as close as possible

• Torsional strain (~12 KJ/mol)

(99%) (1%)

(2.9kcal/mol)

Decane

Drawing Chemical Structures

Skeletalstructure

Cycloalkanes

• General formula: (CH2)n or CnH2n

Step 1 Find the parent chain

Step 2 Number the substituents, and write the name

Cis-Trans Isomerism in Cycloalkanes

• Cycloalkanes are less flexible than the open-chain alkanes

• No rotation around a C-C bond can take place in cycloalkane without breaking the ring

• Because of their cyclic structure, cycloalkanes have two sides: a “top”side and a “bottom”side

• Isomerism is possible in substituted cycloalkanes

• Constitutional isomers– Have different connections among atoms

• Stereoisomers – Have the same connections– Differ in three-dimensional orientation

• Cis-trans isomers have– a subclass of stereoisomers– the same molecular formula– the same connectivity– an arrangement of atoms in space that cannot be

interconverted by rotation about single bonds under ordinary conditions

• Cis: A prefix meaning on the same side

Trans: A prefix meaning on opposite side

Conformations of Some Cycloalknes

• Cyclopropane– is a flat, triangular molecule with C-C-C bond

angles of 60o – All six C-H bonds have an eclipsed arrangement

with their neighbors

• Intramolecular strain: – Torsional strain: arises when atoms not bonded

to each other are forced abnormally close to each other; e.g., eclipsed hydrogens in ethane

– Angle strain: introduced into a molecule when a bond angle is deformed from its ideal value (109.5o)

– Steric strain: arises when two groups are too close together and try to occupy the same space

• Cyclohexane– is a not flat– All C-C-C bond angles are near 109o – All adjacent C-H bonds are staggered– Three-dimensional shape called a chair

conformation

Axial and Equatorial Bonds in Cyclohexane

Axial and Equatorial Bonds in Cyclohexane

• There are two kinds of positions for There are two kinds of positions for hydrogens on the chair conformation of hydrogens on the chair conformation of cyclohexane cyclohexane – Axial position is perpendicular to the ring– Equatorial position is in the plane of the ring

• Each carbon atom has one axial and one equatorial position

• Each side of the ring has 3 axial and 3 equatorial positions

Conformational mobility of Cyclohexane

• There are two equivalent chair conformations

• Different chair cyclohexane conformations readily interconvert, resulting in the exchange of axial and equatorial positions– all C-H bonds equatorial in one chair are axial in

the other, and vice versa

• For monosubstituted cyclohexane, the substituent is always more stable in an equatorial position than in an axial position

• A steric interference occurs in the axial conformation in methylcyclohexane

• The 1,3-diaxial interaction introduces 7.6 kJ/mol of steric strain into the methylcyclohexane because the axial methyl group and the nearby axial hydrogen are too close together

(more stable)(1,3-diaxial interaction)