Alkanes and Cycloalkanes Unit 4 4-1 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 4-2 Hydrocarbons Hydrocarbons Saturated Class Alkan es Carbon- Only carboncarbon carbon sin gle bonding bond s HH Example H-C-C-H HH Name Ethane Unsaturated Alkenes Alkynes Arenes One or more carbon-carbon double b on ds H H C C H H One or more carbon-carb on triple bonds One or more benzenelike rings Ethene Acetylene H-C C-H Ben zene 4-3 Structure Shape • tetrahedral about carbon • all bond angles are approximately 109.5° 4-4 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 CH 2 CH2 CH3 Propane Butan e Pentan e 4-5 Constitutional Isomerism Constitutional isomers: compounds with the same molecular formula but a different connectivity of their atoms • example: C4H10 CH3 CH2 CH2 CH3 Butane (bp -0.5°C) CH3 CH3 CHCH3 2-Methylpropane (bp -11.6°C) 4-6 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 CH3 CHCH42 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 4-7 1 Constitutional Isomerism Molecular Formula CH4 Constitutional Is omers 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 4-8 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 4-9 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 4-10 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) 4-11 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) 4-12 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 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) 4-13 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) 4-14 Nomenclature - IUPAC Alkyl groups Name Condens ed Structural Formula Name butyl - CH 2 CH2 CH2 CH 3 methyl Condens ed Structural Formula - CH 3 2-methylpropyl (isobutyl) - CH 2 CHCH 3 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 4-15 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 4-16 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 4-17 Cycloalkanes General formula CnH2n • five- and six-membered rings are the most common and most stable 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 4-18 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 4-19 Cycloalkanes Example: name these cycloalkanes (a) (b) (c) (d) 4-20 Bicycloalkanes Bicycloalkane: an alkane that contains two rings that share two carbons Bicyclo[4.4.0]decane (Decalin) Bicyclo[4.3.0]nonane (Hydrindane) Bicyclo[2.2.1]heptane (Norbornane) 4-21 Bicycloalkanes Nomenclature • parent is the alkane of the same number of carbons as are in the rings • number from a bridgehead, along longest bridge back to the bridgehead, then along the next longest bridge, etc. • show the lengths of bridges in brackets, from longest to shortest 1 2 6 7 3 5 4 Bicyclo[2.2.1]h eptane 4-22 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 4-23 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 4-24 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 4-25 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 4-26 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 4-27 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 4-28 Cyclobutane • puckering from planar cyclobutane reduces torsional strain but increases angle strain • the conformation of minimum energy is a puckered “butterfly” conformation • strain energy is about 110 kJ (26.3 kcal)/mol 4-29 Cyclopentane • puckering from planar cyclopentane reduces torsional strain, but increases angle stain • the conformation of minimum energy is a puckered “envelope” conformation • strain energy is about 42 kJ (6.5 kcal)/mol 4-30 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 4-31 Cyclohexane In a chair conformation, six H are equatorial and six are axial 4-32 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 4-33 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 4-34 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 or a carbon-carbon double bond 4-35 Isomers relationships among isomers 4-36 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 4-37 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 4-38 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) 4-39 Cis,Trans Isomerism cis-1,4-Dimethylcyclohexane H H CH3 H3 C H H CH3 conformation s are of equal s tability CH3 4-40 Cis,Trans Isomerism The decalins H H A B H trans-Decalin H H H A B H H cis-Decalin 4-41 Bicycloalkanes Norbornane drawn from three different perspectives one carbon b rid ge betw een carbons 1 and 4 of the s ix-membered ring 1 1 2 6 1 4 1 3 5 4 (a) 2 3 5 4 (b ) O 6 (c) 4 Camp hor 4-42 Bicycloalkanes Adamantane join the three axial bonds to a CH group add three axial b on ds Cyclohexane Ad aman tane NH2 Amantadin e (antiviral agent) 4-43 Physical Properties Intermolecular • • • • forces of attraction (example) ion-ion (Na+ and Cl- in NaCl) ion-dipole (Na+ and Cl- solvated in aqueous solution) dipole-dipole and hydrogen bonding dispersion forces (very weak electrostatic attraction between temporary dipoles) 4-44 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 4-45 Physical Properties Constitutional isomers have different physical properties mp (°C) bp (°C) Density (g/mL) -95 68.7 0.659 -154 60.3 0.653 2,3-dimethylbutane -118 -129 63.3 58.0 0.664 0.661 2,2-dimethylbutane -98 49.7 0.649 Name hexane 2-methylpentane 3-methylpentane 4-46 Oxidation of Alkanes 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 0 dioxide and water H 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) 4-47 Heat of Combustion Heat of combustion for constitutional isomers Hydrocarbon S tru ctural formu la H 0 [k J (k cal)/mol] Octane -5470.6 (-1307.5) 2-Methylhep tane -5465.6 (-1306.3) 2,2-D imeth ylhexan e -5458.4 (1304.6) 2,2,3,3-Tetramethylbu tane -5451.8 (1303.0) 4-48 Heats of Combustion For constitutional isomers [kJ (kcal)/mol] -5470.6 (-1307.5) -5465.6 (-1306.3) -5458.4 (1304.6)-5451.8 (1303.0) 8 CO2 + 9 H2 O 4-49 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 4-50 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) 4-51