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ORGANIC CHEMISTRY 171 Section 201 2-1 Alkanes and Cycloalkanes Chapter 2 2-2 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 2-3 Hydrocarbons Hydrocarb on s Saturated Class Carboncarbon bondin g Example N ame Un saturated Alkan es (Ch apter 2) Alk enes (Chap ters 5-6) A lkynes (Ch apter 7) Arenes (Ch apter 21-22) Only carboncarbon sin gle bond s HH H-C-C-H HH Ethan e One or more carb on -carbon double b on ds H H C C H H One or more carbon-carb on trip le bonds On e or more ben zenelike rin gs Eth ene Acetylen e H-C C-H Ben zene 2-4 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. 2-5 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. 6 2-6 Organic Molecules and Functional Groups Functional Groups: Heteroatoms 7 2-7 Functional Groups: Carbonyl groups 8 2-8 Alcohols Contain an -OH (hydroxyl) group bonded to a tetrahedral carbon atom. H H : -C-O-H : H-C-C-O-H Fu nctional group Ethanol H H Ethan ol (an alcohol) may also be written as a condensed structural formula. CH3 -CH2 -OH or CH3 CH2 OH 2-9 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. H CH3 -C-OH H A 1° alcohol H CH3 -C-OH CH3 A 2° alcohol CH3 CH3 -C-OH CH3 A 3° alcohol 2-10 Alcohols There are two alcohols with molecular formula C3H8O HHH H-C-C-C-O-H or CH3 CH2 CH2 OH H HH a 1° alcohol H HOH H C-C-C-H HH H or OH CH3 CHCH3 a 2° alcohol 2-11 Amines an amino group; an sp3-hybridized nitrogen bonded to one, two, or three carbon atoms. Contain • An amine may by 1°, 2°, or 3°. Methylamine (a 1° amine) CH3 N H CH3 Dimethylamine (a 2° amine) : H : : CH3 N H CH3 N CH3 CH3 Trimethylamine (a 3° amine) 2-12 Aldehydes and Ketones Contain : O: C a carbonyl (C=O) group. O H CH3 -C- H Functional Acetaldehyde (an aldehyde) group :O: O C CH3 -C- CH3 Functional Acetone (a ketone) group 2-13 Carboxylic Acids Contain : Fu nctional group :O: CH3 -C-O-H : O C O H a carboxyl (-COOH) group. or CH3 COOH or CH3 CO2 H Acetic aci d (a carboxy li c acid ) 2-14 Carboxylic Esters Ester: A derivative of a carboxylic acid in which the carboxyl hydrogen is replaced by a carbon group. O C O Functional group O CH3 - C-O- CH 2 -CH3 Ethyl acetate (an ester) 2-15 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. O C N Fu nctional group O CH3 -C-N-H H Acetamid e (a 1° amid e) • The six atoms of the amide functional group lie in a plane with bond angles of approximately 120°. 2-16 Alkanes 2-17 Structure Shape • tetrahedral about carbon • all bond angles are approximately 109.5° • sp3 hybridization 2-18 Drawing Alkanes Line-angle formulas • an abbreviated way to draw structural formulas • each vertex and line ending represents a carbon Ball-and stick mod el Line-an gle formula Structu ral formu la CH3 CH2 CH3 CH3 CH2 CH2 CH3 CH3 CH2 CH2 CH2 CH3 Propane Butan e Pentan e 2-19 Constitutional Isomerism Constitutional isomers: compounds with the same molecular formula but a different connectivity of their atoms • example: C4H10 CH3 CH2 CH 2 CH3 Butane (bp -0.5°C) CH3 CH3 CHCH3 2-Methylpropane (bp -11.6°C) 2-20 Constitutional Isomerism • do these formulas represent constitutional isomers? CH3 CH3 CH3 CH3 CHCH2 CH and CH3 CH2 CHCHCH3 CH3 CH3 (each is C7 H 16) • 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 CH3 5 CH3 4 CH3 CHCH2 CH 1 2 3 CH3 1 2 3 4 and 5 CH3 CH3 CH2 CHCHCH3 2 1 CH3 5 4 3 5 4 3 2 2-21 1 Constitutional Isomerism Molecular Formula CH4 Cons titutional Isomers C5 H1 2 1 3 C1 0 H2 2 75 C1 5 H3 2 4,347 C2 5 H5 2 36,797,588 C3 0 H6 2 4,111,846,763 World population is about 6,000,000,000 2-22 Nomenclature - IUPAC Suffix -ane specifies an alkane Prefix tells the number of carbon atoms Prefix Carbons meth1 eth2 prop3 but4 pent5 hex6 7 heptoct8 non9 dec10 Carbons Prefix undec11 dodec12 tridec13 tetradec14 pentadec15 hexadec16 heptadec17 octadec18 nonadec19 eicos 20 2-23 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 RAlkane Name Alkyl group Name CH4 Methane CH3 - Methyl group CH3 CH3 Ethane CH3 CH2 - Ethyl group 2-24 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 CH3 CH3 CHCH3 1 2 3 2-Methylprop ane 4. If there is one substituent, number the chain from the end that gives it the lower number CH3 CH3 CH2 CH2 CHCH3 4 5 32 1 2-Methylpentane 2 1 34 5 (not 4-methylpentane) 2-25 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 6 5 4 2 3 1 1 2 3 5 4 6 2,4-Dimethylhexan e (n ot 3,5-d imethylhexan e) 2-26 Nomenclature - I UPAC 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 1 2 3 4 5 6 7 3-Ethyl-5-methylh eptane 7 6 5 4 3 2 1 (n ot 3-methyl-5-ethylheptane) 2-27 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 1 2 3 4 5 6 4-Ethyl-2,2-dimethylh exane (not 2,2-dimethyl-4-eth ylh exane) 2-28 Nomenclature - IUPAC Alkyl groups Name Condens ed Structural Formula butyl - CH 2 CH2 CH2 CH 3 Name methyl Condens ed Structural Formula - CH 3 2-methylpropyl (isobutyl) - CH 2 CHCH3 ethyl - CH 2 CH3 propyl - CH 2 CH2 CH3 1-methylpropyl (sec- butyl) - CH CH 2 CH3 1-methylethyl (isopropyl) - CH CH 3 CH3 CH3 CH3 CH3 1,1-dimethylethyl (tert- butyl) - CCH3 CH3 2-29 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; neothat it terminates in -C(CH3)3 CH3 CH3 CH2 CH2 CH3 Butane CH3 CHCH3 Is ob utane CH3 CH3 CH2 CH2 CH2 CH3 CH3 CH2 CHCH3 Pentan e Is op entane CH3 CH3 CCH3 CH3 N eopentan e 2-30 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 2-31 Cycloalkanes General formula CnH2n • five- and six-membered rings are the most common Structure and nomenclature • to name, prefix the name of the corresponding openchain 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 2-32 Cycloalkanes Line-angle drawings • each line represents a C-C bond • each vertex and line ending represents a C C C C C C C C C H2 C H2 C CH2 CH3 CH CH CH2 CH3 C8 H1 6 2-33 Cycloalkanes Example: name these cycloalkanes (a) (b) (c) (d) 2-34 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 Infix -an-en-yn- Nature of Carbon-Carbon Bonds in the Parent Chain Suffix Class hydrocarbon all single bonds -e -ol alcohol -al aldehyde -amine amine -one ketone -oic acid carboxylic acid one or more double bonds one or more triple bonds 2-35 IUPAC - General prop-en-e = propene CH3 CH=CH 2 eth-an-ol = ethanol OH but-an-one = butanone HC CH but-an-al = butanal pent-an-oic acid = pentanoic acid CH3 CH2 NH 2 cyclohex-an-ol = cyclohexanol O eth-yn-e = ethyne CH3 CH2 CH2 CH eth-an-amine = ethanamine O O CH3 CCH2 CH 3 CH3 CH2 OH CH3 CH2 CH2 CH2 COH 2-36 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 2-37 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 H H H H H H 2-38 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 H H H H HH 2-39 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 +12.6 kJ/mol 2-40 Conformations angle (Q): the angle created by two intersecting planes Dihedral 2-41 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 2-42 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 H H H H H H 2-43 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 2-44 Cyclohexane In a chair conformation, six H are equatorial and six are axial 2-45 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 2-46 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 2-47 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 2-48 Methylcyclohexane Equatorial and axial methyl conformations CH3 +7.28 kJ/mol CH3 2-49 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) 2-50 Cis,Trans Isomers 1,2-Dimethylcyclopentane H H H H H H H H H CH3 H3 C CH3 CH3 cis-1,2-D imeth ylcyclop entane H H H HH3 C H H CH3 H3 C H CH3 trans-1,2-D imethylcyclop entane 2-51 Cis,Trans Isomerism 1,4-Dimethylcyclohexane H H3 C CH3 H H H3 C CH3 H3 C t rans-1,4-D imethylcyclohexane H CH3 CH3 H3 C cis-1,4-D imethylcycloh exane 2-52 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 CH3 H H H3 C H CH3 CH3 (less s table) H (more s table) 2-53 Cis,Trans Isomerism cis-1,4-Dimethylcyclohexane H H CH3 H3 C H H CH3 conformation s are of equal s tability CH3 2-54 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 2-55 Physical Properties Constitutional isomers have different physical properties Name mp (°C) bp (°C) Density (g/mL) hexane 2-methylpentane 3-methylpentane 2,3-dimethylbutane 2,2-dimethylbutane -95 -154 -118 -129 -98 68.7 60.3 63.3 58.0 49.7 0.659 0.653 0.664 0.661 0.649 2-56 Preparation and Reactions of Alkanes 2-57 Preparation of Alkanes via Reduction 1) Hydrogenation of Alkenes Hydrogenation of Alkynes R R' C C R R H 2 /Pt R' C C R' R R' CH CH R H 2 /Pt R' H H R C C R' H H 2-58 2. 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 2-59 CH3 CH3 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 2-60 b) 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 SnCl2 n-butane CH3 CH3 CH3CCH3 + Zn/H+ CH3CHCH3 + ZnBr2 Br tert-butyl bromide isobutane 2-61 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.) 2-62 Alkane, reactions: 1. Halogenation 2. Combustion (oxidation) 3. Pyrolysis (cracking) 2-63 1. 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 2-64 CH3CH3 + Cl2, hv CH3CH2-Cl + HCl ethane ethyl chloride CH3CH2CH3 + Cl2, hv CH3CH2CH2-Cl + CH3CHCH3 propane n-propyl chloride Cl 45% isopropyl chloride 55% possible gives a mixture of both the alkyl halides! 2-65 CH3CH2CH2CH3 + Cl2, hv CH3CH2CH2CH2-Cl n-butane n-butyl chloride 28% + CH3CH2CHCH3 Cl 72% sec-butyl chloride CH3 CH3 CH3CHCH3 + Cl2, hv CH3CHCH2-Cl isobutane 64% isobutyl chloride + CH3 CH3CCH3 Cl 36% tert-butyl chloride 2-66 chlorination of propane, mechanism: 1) 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• 2-67 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 H 0 kJ(k cal)/mol CH4 + 2 O2 Methan e CH3 CH2 CH3 + 5 O2 Propane CO2 + 2 H2 O -890.4 (-212.8) 3 CO2 + 4 H2 O -2220 (-530.6) 2-68 Sources of Alkanes Natural gas • 90-95% methane Petroleum • • • • • • 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 2-69 Gasoline Octane rating: the percent 2,2,4-trimethylpentane (isooctane) in a mixture of isooctane and heptane that has equivalent antiknock properties Hep tane (octane rating 0) 2,2,4-Trimeth ylp entane (octane rating 100) 2-70 Synthesis Gas A mixture of carbon monoxide and hydrogen in varying proportions which depend on the means by which it is produced C + Coal CH4 + Methane H2 O 1 O2 2 heat catalyst CO + H2 CO + 2 H2 2-71 Synthesis Gas Synthesis gas is a feedstock for the industrial production of methanol and acetic acid CO + 2 H2 catalyst CH3 OH Methanol CH3 OH + CO Methanol catalyst O CH3 COH Acetic acid • it is likely that industrial routes to other organic chemicals from coal via methanol will also be developed 2-72 Alkanes and Cycloalkanes End Chapter 2 2-73