ORGANIC CHEMISTRY 171 Section 201

Alkanes and Cycloalkanes Chapter 2

Structure Hydrocarbon: a compound composed only of carbon and hydrogen Saturated hydrocarbon: a hydrocarbon containing only single bonds Alkane: a saturated hydrocarbon whose carbons are arranged in an open chain Aliphatic hydrocarbon: another name for an alkane

Hydrocarbons

Functional Groups Functional group: An atom or group of atoms within a molecule that shows a characteristic set of physical and chemical properties. Functional groups are important for three reasons: 1. Allow us to divide compounds into classes. 2. Each group undergoes characteristic chemical reactions. 3. Provide the basis for naming compounds.

Organic Molecules and Functional Groups Functional Groups: Hydrocarbons Hydrocarbons are compounds made up of only the elements carbon and hydrogen. They may be aliphatic or aromatic.

Organic Molecules and Functional Groups Functional Groups: Heteroatoms

Functional Groups: Carbonyl groups

Alcohols Contain an -OH (hydroxyl) group bonded to a tetrahedral carbon atom. Ethanol may also be written as a condensed structural formula.

Alcohols Alcohols are classified as primary (1°), secondary (2°), or tertiary (3°) depending on the number of carbon atoms bonded to the carbon bearing the -OH group.

Alcohols There are two alcohols with molecular formula C3H8O

Amines Contain an amino group; an sp3-hybridized nitrogen bonded to one, two, or three carbon atoms. An amine may by 1°, 2°, or 3°.

Aldehydes and Ketones Contain a carbonyl (C=O) group.

Carboxylic Acids Contain a carboxyl (-COOH) group.

Carboxylic Esters Ester: A derivative of a carboxylic acid in which the carboxyl hydrogen is replaced by a carbon group.

Carboxylic Amide Carboxylic amide, commonly referred to as an amide: A derivative of a carboxylic acid in which the -OH of the -COOH group is replaced by an amine. The six atoms of the amide functional group lie in a plane with bond angles of approximately 120°.

Alkanes

Structure Shape sp3 hybridization tetrahedral about carbon all bond angles are approximately 109.5° sp3 hybridization

Drawing Alkanes Line-angle formulas an abbreviated way to draw structural formulas each vertex and line ending represents a carbon

Constitutional Isomerism Constitutional isomers: compounds with the same molecular formula but a different connectivity of their atoms example: C4H10

Constitutional Isomerism do these formulas represent constitutional isomers? find the longest carbon chain number each chain from the end nearest the first branch compare chain lengths as well the identity and location of branches

Constitutional Isomerism World population is about 6,000,000,000

Nomenclature - IUPAC Suffix -ane specifies an alkane Prefix tells the number of carbon atoms

Nomenclature - IUPAC Parent name: the longest carbon chain Substituent: a group bonded to the parent chain alkyl group: a substituent derived by removal of a hydrogen from an alkane; given the symbol R-

Nomenclature - IUPAC 1.The name of a saturated hydrocarbon with an unbranched chain consists of a prefix and suffix 2. The parent chain is the longest chain of carbon atoms 3. Each substituent is given a name and a number 4. If there is one substituent, number the chain from the end that gives it the lower number

Nomenclature - IUPAC 5. If there are two or more identical substituents, number the chain from the end that gives the lower number to the substituent encountered first indicate the number of times the substituent appears by a prefix di-, tri-, tetra-, etc. use commas to separate position numbers

Nomenclature - IUPAC 6. If there are two or more different substituents, list them in alphabetical order number from the end of the chain that gives the substituent encountered first the lower number

Nomenclature - IUPAC 7. The prefixes di-, tri-, tetra-, etc. are not included in alphabetization alphabetize the names of substituents first and then insert these prefixes

Nomenclature - IUPAC Alkyl groups

Nomenclature - Common The number of carbons in the alkane determines the name all alkanes with four carbons are butanes, those with five carbons are pentanes, etc. iso- indicates the chain terminates in -CH(CH3)2; neo- that it terminates in -C(CH3)3

Classification of C & H Primary (1°) C: a carbon bonded to one other carbon 1° H: a hydrogen bonded to a 1° carbon Secondary (2°) C: a carbon bonded to two other carbons 2° H: a hydrogen bonded to a 2° carbon Tertiary (3°) C: a carbon bonded to three other carbons 3° H: a hydrogen bonded to a 3° carbon Quaternary (4°) C: a carbon bonded to four other carbons

Cycloalkanes General formula CnH2n Structure and nomenclature five- and six-membered rings are the most common Structure and nomenclature to name, prefix the name of the corresponding open-chain alkane with cyclo-, and name each substituent on the ring if only one substituent, no need to give it a number if two substituents, number from the substituent of lower alphabetical order if three or more substituents, number to give them the lowest set of numbers and then list substituents in alphabetical order

Cycloalkanes Line-angle drawings each line represents a C-C bond each vertex and line ending represents a C

Cycloalkanes Example: name these cycloalkanes

IUPAC - General prefix-infix-suffix prefix tells the number of carbon atoms in the parent infix tells the nature of the carbon-carbon bonds suffix tells the class of compound Nature of Carbon-Carbon Bonds in the Parent Chain Suffix Class Infix -e hydrocarbon -an- all single bonds -ol alcohol -en- one or more double bonds -al aldehyde -yn- one or more triple bonds -amine amine -one ketone -oic acid carboxylic acid

IUPAC - General prop-en-e = propene eth-an-ol = ethanol but-an-one = butanone but-an-al = butanal pent-an-oic acid = pentanoic acid cyclohex-an-ol = cyclohexanol eth-yn-e = ethyne eth-an-amine = ethanamine

Conformations Conformation: any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond Newman projection: a way to view a molecule by looking along a carbon-carbon single bond

Conformations Staggered conformation: a conformation about a carbon-carbon single bond in which the atoms or groups on one carbon are as far apart as possible from the atoms or groups on an adjacent carbon

Conformations Eclipsed conformation: a conformation about a carbon-carbon single bond in which the atoms or groups of atoms on one carbon are as close as possible to the atoms or groups of atoms on an adjacent carbon

Conformations Torsional strain also called eclipsed interaction strain strain that arises when nonbonded atoms separated by three bonds are forced from a staggered conformation to an eclipsed conformation the torsional strain between eclipsed and staggered ethane is approximately 12.6 kJ (3.0 kcal)/mol

Conformations Dihedral angle (Q): the angle created by two intersecting planes

Conformations Steric strain (nonbonded interaction strain): the strain that arises when atoms separated by four or more bonds are forced closer to each other than their atomic (contact) radii will allow Angle strain: strain that arises when a bond angle is either compressed or expanded compared to its optimal value

Cyclopropane angle strain: the C-C-C bond angles are compressed from 109.5° to 60° torsional strain: there are 6 sets of eclipsed hydrogen interactions strain energy is about 116 kJ (27.7 kcal)/mol

Cyclohexane Chair conformation: the most stable puckered conformation of a cyclohexane ring all bond C-C-C bond angles are 110.9° all bonds on adjacent carbons are staggered

Cyclohexane In a chair conformation, six H are equatorial and six are axial

Cyclohexane For cyclohexane, there are two equivalent chair conformations all C-H bonds equatorial in one chair are axial in the alternative chair, and vice versa

Cyclohexane Boat conformation: a puckered conformation of a cyclohexane ring in which carbons 1 and 4 are bent toward each other there are four sets of eclipsed C-H interactions and one flagpole interaction a boat conformation is less stable than a chair conformation by 27 kJ (6.5 kcal)/mol

Cyclohexane Twist-boat conformation approximately 41.8 kJ (5.5 kcal)/mol less stable than a chair conformation approximately 6.3 kJ (1.5 kcal)/mol more stable than a boat conformation

Methylcyclohexane Equatorial and axial methyl conformations

Cis,Trans Isomerism Stereoisomers: compounds that have the same molecular formula the same connectivity a different orientation of their atoms in space Cis,trans isomers stereoisomers that are the result of the presence of either a ring (this chapter) or a carbon-carbon double bond (Chapter 5)

Cis,Trans Isomers 1,2-Dimethylcyclopentane

Cis,Trans Isomerism 1,4-Dimethylcyclohexane

Cis,Trans Isomerism trans-1,4-Dimethylcyclohexane the diequatorial-methyl chair conformation is more stable by approximately 2 x (7.28) = 14.56 kJ/mol

Cis,Trans Isomerism cis-1,4-Dimethylcyclohexane

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

Physical Properties Constitutional isomers have different physical properties

Preparation and Reactions of Alkanes

Preparation of Alkanes via Reduction 1) Hydrogenation of Alkenes Hydrogenation of Alkynes

Reduction of an alkyl halide a) hydrolysis of a Grignard reagent (two steps) i) R—X + Mg  RMgX (Grignard reagent) ii) RMgX + H2O  RH + Mg(OH)X SB SA WA WB CH3CH2CH2-Br + Mg  CH3CH2CH2-MgBr n-propyl bromide n-propyl magnesium bromide CH3CH2CH2-MgBr + H2O  CH3CH2CH3 + Mg(OH)Br propane

CH3CH-Br + Mg  CH3CH-MgBr isopropyl bromide isopropyl magnesium bromide CH3 CH3CH-MgBr + H2O  CH3CH2CH3 propane CH3CH2CH2-MgBr + D2O  CH3CH2CH2D heavy water CH3 CH3 CH3CH-MgBr + D2O  CH3CHD

with an active metal and an acid R—X + metal/acid  RH active metals = Sn, Zn, Fe, etc. acid = HCl, etc. (H+) CH3CH2CHCH3 + Sn/HCl  CH3CH2CH2CH3 + Cl sec-butyl chloride n-butane CH3 CH3 CH3CCH3 + Zn/H+  CH3CHCH3 + ZnBr2 Br tert-butyl bromide isobutane SnCl2

Reactions of alkanes: alkane + H2SO4  no reaction (NR) alkane + NaOH  NR alkane + Na  NR alkane + KMnO4  NR alkane + H2,Ni  NR alkane + Br2  NR alkane + H2O  NR (Alkanes are typically non-reactive. They don’t react with acids, bases, active metals, oxidizing agents, reducing agents, halogens, etc.)

Alkane, reactions: Halogenation 2. Combustion (oxidation) 3. Pyrolysis (cracking)

Halogenation R-H + X2, heat or hv  R-X + HX a) heat or light required for reaction. b) X2: Cl2 > Br2  I2 c) yields mixtures  d) H: 3o > 2o > 1o > CH4 e) bromine is more selective

CH3CH3 + Cl2, hv  CH3CH2-Cl + HCl ethane ethyl chloride CH3CH2CH3 + Cl2, hv  CH3CH2CH2-Cl + CH3CHCH3 propane n-propyl chloride Cl isopropyl chloride 45% 55% gives a mixture of both the possible alkyl halides! 

CH3CH2CH2CH3 + Cl2, hv  CH3CH2CH2CH2-Cl + CH3CH2CHCH3 72% Cl n-butane n-butyl chloride 28% + CH3CH2CHCH3 72% Cl sec-butyl chloride CH3 CH3 CH3CHCH3 + Cl2, hv  CH3CHCH2-Cl 64% isobutane isobutyl chloride CH3 CH3CCH3 36% tert-butyl chloride

chlorination of propane, mechanism: abstraction of 1o hydrogen: Cl• + CH3CH2CH3  CH3CH2CH2• + HCl or abstraction of 2o hydrogen: Cl• + CH3CH2CH3  CH3CHCH3 + HCl • 2)CH3CH2CH2• + Cl2  CH3CH2CH2Cl + Cl• or CH3CHCH3 + Cl2  CH3CHCH3 + Cl• • Cl plus terminating steps 3) Cl—Cl  2 Cl•

2.Oxidation of Alkanes ( combustion ) Oxidation is the basis for their use as energy sources for heat and power heat of combustion: heat released when one mole of a substance in its standard state is oxidized to carbon dioxide and water

Sources of Alkanes Natural gas Petroleum Coal 90-95% methane gases (bp below 20°C) naphthas, including gasoline (bp 20 - 200°C) kerosene (bp 175 - 275°C) fuel oil (bp 250 - 400°C) lubricating oils (bp above 350°C) asphalt (residue after distillation) Coal

Gasoline Octane rating: the percent 2,2,4-trimethylpentane (isooctane) in a mixture of isooctane and heptane that has equivalent antiknock properties

Synthesis Gas A mixture of carbon monoxide and hydrogen in varying proportions which depend on the means by which it is produced

Synthesis Gas Synthesis gas is a feedstock for the industrial production of methanol and acetic acid it is likely that industrial routes to other organic chemicals from coal via methanol will also be developed

Alkanes and Cycloalkanes End Chapter 2