Uploaded by
Asnake Tsegaye
Alkanes and Cycloalkanes: Structure, Nomenclature, Isomerism
advertisement
Chapter 3: Alkanes and Cycloalkanes © 2005 John Wiley & Sons, Inc. All rights reserved 3-1 Organic Chemistry Hydrocarb on s Satu rated Class Alkan es (Ch apter 3) Carb on - Only carboncarbon carb on single bond ing bonds HH Example H-C-C-H HH N ame Ethan e © 2005 John Wiley & Sons, Inc. All rights reserved Un saturated Alk enes (Ch apters 4-5) Alkynes (Chap ter 4) Arenes (Ch apter 9) One or more carb on -carbon doub le bonds H H C C H H Ethylene On e or more carb on -carbon triple b on ds On e or more ben zenelike rin gs H-C C-H Acetylene Benzen e 3-2 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 a open chain • Aliphatic hydrocarbon: another name for an alkane © 2005 John Wiley & Sons, Inc. All rights reserved 3-3 Structure • Shape • tetrahedral about carbon • all bond angles are approximately 109.5° © 2005 John Wiley & Sons, Inc. All rights reserved 3-4 Representing Alkanes Ball-an d-stick model Line-angle formula Stru ctural formula © 2005 John Wiley & Sons, Inc. All rights reserved CH3 CH2 CH2 CH3 CH3 CH2 CH2 CH2 CH3 Butane Pen tane 3-5 Nomenclature • Alkanes have the general formula CnH2n+2 • names of unbranched chain alkanes N ame Molecular Formula methane CH 4 ethan e C2 H 6 propane C H 3 bu tane 8 C4 H 1 0 pen tane C5 H 1 2 hexan e C6 H 1 4 hep tane C7 H 1 6 octane © 2005 John Wiley & Sons, Inc. All rights reserved C8 H1 8 N ame Molecular Formula nonane C9 H2 0 decan e C1 0 H 2 2 dodecan e C1 2 H 2 6 tetrad ecane C1 4 H 3 0 hexadecane C1 6 H 3 4 octadecan e C1 8 H 3 8 eicosane C2 0 H 4 2 3-6 Constitutional Isomers • Constitutional isomers: compounds with the same molecular formula but a different connectivity of their atoms • there are two constitutional isomers with molecular formula C4H10 CH3 © 2005 John Wiley & Sons, Inc. All rights reserved CH3 CH2 CH2 CH3 CH3 CHCH3 Butane (bp -0.5°C) 2-Meth ylp ropan e (bp -11.6°C) 3-7 Constitutional Isomerism • the potential for constitutional isomerism is enormous Molecular Formul a CH 4 Constitutional 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 © 2005 John Wiley & Sons, Inc. All rights reserved World population is about 6,000,000,000 3-8 IUPAC Nomenclature • suffix -ane specifies an alkane • prefix tells the number of carbon atoms Prefix Carbons meth1 eth 2 prop3 but4 pent5 hex6 hept7 oct8 non 9 dec10 © 2005 John Wiley & Sons, Inc. All rights reserved Carb on s Prefix undec11 dod ec12 tridec13 tetradec14 pentadec- 15 hexadec16 heptadec- 17 octad ec18 non adec19 eicos 20 3-9 IUPAC Nomenclature • 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• CH4 becomes CH3- (methyl) • CH3CH3 becomes CH3CH2- (ethyl) subs tituent parent chain 1 2 3 4 5 6 8 7 4-Meth yloctan e © 2005 John Wiley & Sons, Inc. All rights reserved 3-10 IUPAC Nomenclature • Common alkyl groups N ame methyl Con dens ed Structu ral Formula -CH3 ethyl -CH2 CH3 propyl -CH2 CH2 CH3 isopropyl -CHCH3 CH3 bu tyl -CH2 CH2 CH2 CH3 © 2005 John Wiley & Sons, Inc. All rights reserved N ame isobu tyl sec-butyl Con dens ed Structu ral Formula -CH2 CHCH3 CH3 -CHCH2 CH3 CH3 CH3 t ert-bu tyl -CCH3 CH3 3-11 IUPAC Nomenclature 1.The name of an alkane with an unbranched chain consists of a prefix and the suffix ane 2. For branched alkanes, the parent chain is the longest chain of carbon atoms 3. Each substituent is given a name and a number CH3 CH3 CHCH3 2-Methylp ropan e 2 1 3 4. If there is one substituent, number the chain from the end that gives it the lower number CH3 CH3 CH2 CH2 CHCH3 2-Methylpen tane © 2005 John Wiley & Sons, Inc. All rights reserved 5 4 3 2 1 3-12 IUPAC Nomenclature 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 CH3 CH3 CH2 CHCH2 CHCH2 CH3 CH2 CH3 2 1 4 3 6 5 7 3-Ethyl-5-meth ylh eptane © 2005 John Wiley & Sons, Inc. All rights reserved 3-13 IUPAC Nomenclature 7. The prefixes di-, tri-, tetra-, etc. are not included in alphabetization CH3 CH2 CH3 CH3 CCH2 CHCH2 CH3 CH3 1 2 3 4 6 5 4-Ethyl-2,2-dimeth ylh exane © 2005 John Wiley & Sons, Inc. All rights reserved 3-14 Classification of Carbons • • • • Primary (1°): a C bonded to one other carbon Secondary (2°): a C bonded to two other carbons Tertiary (3°): a C bonded to three other carbons Quaternary (4°): a C bonded to four other carbons a 2° carb on a 4° carb on a 3° carb on CH3 CH3 -C-CH2 -CH-CH3 a 1° carb on CH3 CH3 a 1° carb on 2,2,4-Trimeth ylp entane © 2005 John Wiley & Sons, Inc. All rights reserved 3-15 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 © 2005 John Wiley & Sons, Inc. All rights reserved 3-16 Cycloalkanes • Commonly written as line-angle formulas • examples: Isopropylcyclopentan e 1-tert -Bu tyl-4-methylcycloh exane 1-Isobutyl-2-meth ylcyclohexan e 1-Ethyl-1-methylcyclopropane © 2005 John Wiley & Sons, Inc. All rights reserved 3-17 IUPAC- A General System • 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 Infi x Nature of Carbon-Carbon Bonds in the Parent Chain Suffix Cl ass -e hydrocarbon -an-en- all singl e bonds -ol alcohol one or more double bonds -al aldehyde -yn- one or more triple bonds -amine -one amine -oic acid © 2005 John Wiley & Sons, Inc. All rights reserved ketone carboxyl ic acid 3-18 IUPAC - a general system 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 cyclohex-an-ol = cyclohexanol CH3 CH2 NH 2 eth-yn-e = ethyne O eth-an-amine = ethanamine H O CH3 CH2 OH O OH © 2005 John Wiley & Sons, Inc. All rights reserved 3-19 Conformation • Conformation: any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond • staggered conformation: a conformation about a carbon-carbon single bond where the atoms on one carbon are as far apart as possible from the atoms on an adjacent carbon H H H H H H © 2005 John Wiley & Sons, Inc. All rights reserved 3-20 Conformation • Eclipsed conformation: a conformation about a carboncarbon single bond in which the atoms on one carbon are as close as possible to the atoms on an adjacent carbon H H H H HH • the lowest energy conformation of an alkane is a fully staggered conformation © 2005 John Wiley & Sons, Inc. All rights reserved 3-21 Conformations • Torsional strain: strain that arises when atoms separated by 3 bonds are forced from a staggered conformation to an eclipsed conformation • also called eclipsed interaction strain • the torsional strain between staggered and eclipsed ethane is approximately 3.0 kcal (12.6 kJ)/ mol +3.0 kcal/mol © 2005 John Wiley & Sons, Inc. All rights reserved 3-22 Cycloalkanes • Cyclopentane • in planar cyclopentane, all C-C-C bond angles are 108°, which differ only slightly from the tetrahedral angle of 109.5° • consequently there is little angle strain • angle strain: strain that arises when a bond angle is either compressed or expanded compared with its optimal value © 2005 John Wiley & Sons, Inc. All rights reserved 3-23 Cycloalkanes • Cyclopentane (cont’d) • in planar cyclopentane, there are 10 fully eclipsed C-H bonds, which create torsional strain of approximately 10 kcal/mol • puckering to an “envelope” conformation relieves part of this strain • in an envelope conformation, eclipsed interactions are reduced but angle strain is increased slightly (105°) © 2005 John Wiley & Sons, Inc. All rights reserved 3-24 Cycloalkanes • Cyclohexane • the most stable conformation is a puckered chair conformation • in a chair conformation, all bond angles are approx. 109.5°, and all bonds on adjacent carbons are staggered © 2005 John Wiley & Sons, Inc. All rights reserved 3-25 Cycloalkanes • Chair cyclohexane • six H are equatorial and six H are axial • an axial bond extends from the ring parallel to the imaginary axis of the ring • equatorial bonds are roughly perpendicular to the imaginary axis of the ring © 2005 John Wiley & Sons, Inc. All rights reserved 3-26 Cyclohexane • chair cyclohexane (cont’d) • there are two equivalent chair conformations • all C-H bonds equatorial in one chair are axial in the alternative chair, and vice versa © 2005 John Wiley & Sons, Inc. All rights reserved 3-27 Cyclohexane • Boat conformation: a puckered conformation in which carbons 1 and 4 are bent toward each other • a boat conformation is less stable than a chair conformation by 6.5 kcal (27 kJ)/mol • torsional strain is created by four sets of eclipsed hydrogen interactions • steric strain (nonbonded interaction strain) is created by one set of flagpole interactions © 2005 John Wiley & Sons, Inc. All rights reserved 3-28 Cyclohexane • the alternative chair conformations interconvert via a boat conformation flip th is end dow n flip th is end up © 2005 John Wiley & Sons, Inc. All rights reserved 3-29 Cyclohexane • a group equatorial in one chair is axial in the alternative chair • the two chairs are no longer of equal stability CH3 +1.74 k cal/mol CH3 © 2005 John Wiley & Sons, Inc. All rights reserved 3-30 Cis-trans Isomerism • Cis-trans isomers have • the same molecular formula • the same connectivity of their atoms • an arrangement of atoms in space that cannot be interconverted by rotation about sigma bonds H H H H H H H H H3 C © 2005 John Wiley & Sons, Inc. All rights reserved H H H H3 C H CH 3 H CH3 CH3 cis-1,2-D imethylcyclopentan e H H CH3 H3 C H CH 3 t rans-1,2-D imeth ylcyclopentane 3-31 Cis-trans isomerism • the ring is commonly viewed through an edge or from above H H H H H H H H H3 C CH3 CH3 cis-1,2-D imethylcyclopentan e © 2005 John Wiley & Sons, Inc. All rights reserved H H H H3 C H CH 3 H H H CH3 H3 C H CH 3 t rans-1,2-D imeth ylcyclopentane 3-32 Cis-trans isomerism • cyclohexanes may be viewed as planar hexagons H H3 C CH3 H CH3 H3 C t rans-1,4-D imeth ylcyclohexan e H H3 C H CH3 CH3 H 3 C cis-1,4-D imethylcyclohexane © 2005 John Wiley & Sons, Inc. All rights reserved 3-33 Cis-trans isomerism • or we can represent them as chair conformations • in viewing chair conformations, remember that groups equatorial in one chair are axial in the alternative chair • for trans-1,4-dimethylcyclohexane, the diequatorial chair is more stable than the diaxial chair CH3 axial H equatorial H equ atorial CH3 H CH3 axial CH3 H (less s table) (more stable) t rans -1,4-D imethylcyclohexan e © 2005 John Wiley & Sons, Inc. All rights reserved 3-34 Cis-trans isomerism • for cis-1,4-dimethylcyclohexane, the alternative chairs are of equal stability equ atorial H CH3 H axial CH3 H CH3 H axial CH3 cis-1,4-D imethylcycloh exane (th ese conformation s are of equ al stability) © 2005 John Wiley & Sons, Inc. All rights reserved 3-35 Cis-trans isomerism • Problem: draw the alternative chair conformations of this trisubstituted cyclohexane and state which is the more stable H3 C CH3 CH3 H H H © 2005 John Wiley & Sons, Inc. All rights reserved 3-36 Physical Properties • Alkanes are nonpolar compounds and have only weak interactions between their molecules • Dispersion forces: weak intermolecular forces of attraction resulting from interaction of temporary induced dipoles © 2005 John Wiley & Sons, Inc. All rights reserved 3-37 Physical Properties • Boiling point • low-molecular-weight alkanes (1 to 4 carbons) are gases at room temperature; e.g., methane, propane, butane • higher-molecular-weight alkanes (5 to 17 carbons) are liquids at room temperature (e.g., hexane, decane, gasoline, kerosene) • high-molecular-weight alkanes (18 or more carbons) are white, waxy semisolids or solids at room temperature; e.g., paraffin wax • Density • average density is about 0.7 g/mL • liquid and solid alkanes float on water © 2005 John Wiley & Sons, Inc. All rights reserved 3-38 Physical Properties • Constitutional isomers are different compounds and have different physical properties N ame hexan e 3-methylp entane 2-methylp entane Meltin g Boiling Point Point D en sity (°C) (°C) (g/mL) -95 69 0.659 -6 -23 2,3-dimethylbutan e -129 64 62 58 0.664 0.653 0.662 2,2-dimethylbutan e -100 50 0.649 Hexane 2,2-D imethylbutan e © 2005 John Wiley & Sons, Inc. All rights reserved 3-39 Reactions of Alkanes • Oxidation is the basis for the use of alkanes as energy sources for heat and power • heat of combustion: heat released when one mole of a substance is oxidized to carbon dioxide and water CH4 + 2 O2 Methan e CH3 CH2 CH3 + 5 O2 Propan e © 2005 John Wiley & Sons, Inc. All rights reserved CO2 + 2 H 2 O H° = -212 kcal/mol 3 CO2 + 4 H2 O H° = -530 kcal/mol 3-40 Sources of Alkanes • Natural gas • 90-95% methane, 5-10% ethane • 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 © 2005 John Wiley & Sons, Inc. All rights reserved 3-41 Synthesis Gas • a mixture of carbon monoxide and hydrogen in varying proportions, depending on how it is produced C + H2 O Coal + 1 O2 CH4 heat catalyst 2 CO + H2 CO + 2 H2 Methan e • methanol and acetic acid are produced from synthesis gas catalyst CO + 2 H2 CH 3 OH + CO Methan ol © 2005 John Wiley & Sons, Inc. All rights reserved CH 3 OH Methan ol catalyst O CH 3 COH Acetic acid 3-42 Alkanes and Cycloalkanes End Chapter 3 © 2005 John Wiley & Sons, Inc. All rights reserved 3-43
0
advertisement
Download
advertisement
Add this document to collection(s)
You can add this document to your study collection(s)
Sign in Available only to authorized usersAdd this document to saved
You can add this document to your saved list
Sign in Available only to authorized users