Alkanes and Cycloalkanes PDF

WILLIAM H. BROWN THOMAS POON www.wiley.com/college/brown CHAPTER THREE Alkanes and Cycloalkanes Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-1 Structure Hydrocarbon: A compound composed only of carbon and hydrogen. Saturated hydrocarbon: A hydrocarbon containin...

WILLIAM H. BROWN THOMAS POON www.wiley.com/college/brown CHAPTER THREE Alkanes and Cycloalkanes Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-1 Structure Hydrocarbon: A compound composed only of carbon and hydrogen. Saturated hydrocarbon: A hydrocarbon containing only carbon-carbon single bonds. Alkane: A saturated hydrocarbon whose carbons are arranged in an open chain. Aliphatic hydrocarbon: An alternative name for an alkane. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-2 Organic Chemistry Figure 3.1 The Four Classes of Hydrocarbons Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-3 Structure of Alkanes Shape – Tetrahedral geometry (all carbons are sp3 hybridized). – All bond angles are approximately 109.5°. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-4 Representing Alkanes – Line-angle formula: an abbreviated way to draw structural formulas. Each line represents a single bond. Each line ending represents a CH3 group. Each vertex (angle) represents a carbon atom. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-5 Alkanes Alkanes have the general formula CnH2n+2 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 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 the molecular formula C4H10. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-7 Constitutional Isomerism – The potential for constitutional isomerism from just the elements carbon and hydrogen is enormous. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-8 IUPAC Nomenclature – Suffix -ane specifies an alkane. – Prefix tells the number of carbon atoms. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 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). Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-10 IUPAC Nomenclature Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 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. 4. If there is one substituent, number the chain from the end that gives it the lower number. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-12 IUPAC Nomenclature 5. If there are two or more identical substituents, number the parent chain from the end that gives the lower number to the substituent encountered first. The number of times the substituent occurs is indicated by the prefixes di-, tri-, tetra-, and so on. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-13 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. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-14 IUPAC Nomenclature 7. The prefixes di-, tri-, tetra-, etc. are not included in alphabetization. Iso, as in isopropyl, is included in alphabetization. In the following example, the alphabetizing names are ethyl and methyl. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-15 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. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-16 Cycloalkanes General formula CnH2n – Five- and six-membered rings are the most common. Structure and nomenclature – Prefix the name of the corresponding open-chain alkane with cyclo-, name each substituent on the ring. – If only one substituent, no need to give it a number. – If two substituents, number the ring from the substituent of lower alphabetical order. – If three or more substituents, number the ring to give them the lowest set of numbers, and then list them in alphabetical order. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-17 Cycloalkanes Commonly written as line-angle formulas – examples: Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-18 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. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-19 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. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-20 IUPAC - a general system prop-en-e = propene eth-an-ol = ethanol but-an-one = butanone but-an-al = butanal but-an-oic acid = butanoic acid cyclohex-an-ol = cyclohexanol eth-yn-e = ethyne eth-an-amine = ethanamine Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-21 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. On the right is a Newman projection formula. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-22 Conformation – Eclipsed conformation: A conformation about a carbon-carbon single bond in which the atoms on one carbon are as close as possible to the atoms on an adjacent carbon. – The lowest energy conformation of an alkane is a fully staggered conformation. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-23 Conformations Torsional strain: Strain that arises when nonbonded atoms separated by three 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 12.6 kJ/mol(3.0 kcal/mol). Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-24 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. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-25 Cycloalkanes Cyclopentane (cont'd) – In planar cyclopentane, there are 10 fully eclipsed C- H bonds creating a torsional strain of approximately 42 kJ/mol (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°). Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-26 Cycloalkanes Cyclohexane – The most stable conformation is a puckered conformation called a chair conformation. – In a chair conformation, all bond angles are approx. 109.5° and all bonds on adjacent carbons are staggered. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-27 Cycloalkanes Chair cyclohexane – Six C–H bonds are equatorial and six are axial. – An equatorial bond extends from the ring roughly perpendicular to the imaginary axis of the ring. – An axial bond extends from the ring roughly parallel to the imaginary axis of the ring. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-28 Cyclohexane Boat conformation: A puckered conformation in which carbons 1 & 4 are bent toward each other. – A boat conformation is less stable than a chair conformation by 27 kJ/mol (6.5 kcal/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. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-29 Cyclohexane Following is a structural formula and ball-and- stick model of cholestanol, a close relative of cholesterol. Describe the conformation of each ring in cholestanol. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-30 Cyclohexane chair cyclohexane (cont'd) – There are two equivalent chair conformations. – The alternative chair conformation interconvert via a boat conformation. – All C–H bonds that are equatorial in one chair are axial in the alternative chair, and vice versa. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-31 Methylcyclohexane – A group equatorial in one chair is axial in the alternative chair. – The two chairs are no longer of equal stability. They differ by 7.28 kJ/mol (1.74 kcal/mol) Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-32 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. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-33 Cis-trans isomerism – A cyclopentane ring is commonly viewed through an edge or from above. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-34 Cis-trans isomerism – A cyclohexane ring is commonly viewed as a planar hexagon viewed from the side or from above. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-35 Cis-trans isomerism – Or we can represent a cyclohexane as a chair conformation. – In viewing chair conformations, 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. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-36 Cis-trans isomerism – For cis-1,4-dimethylcyclohexane, the alternative chairs are of equal stability. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-37 Cis-trans isomerism – Problem 3.11: Draw the alternative chair conformations of this trisubstituted cyclohexane, and state which conformation is the more stable. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-38 Conformation of Cyclohexanes Problem: Draw alternative chair conformations of each substituted cycloalkane, and state which conformation of each is the more stable. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-39 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. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-40 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. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-41 Physical Properties Constitutional isomers are different compounds and have different physical properties. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-42 Reactions of Alkanes Oxidation is the basis for the use of alkanes as energy sources for heat and power. – Heat of combustion: the heat released when one mole of a substance is oxidized to carbon dioxide and water. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-43 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 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-44 Sources of Alkanes Figure 3.13 Fractional distillation of petroleum. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-45 Synthesis Gas – Synthesis gas: A mixture of carbon monoxide and hydrogen in varying proportions, depending on how it is produced. – Methanol and acetic acid are produced from synthesis gas. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-46 Methane Economy If the United States moves from a petroleum economy to a natural gas economy as some advocate, synthesis gas and synthesis gas-derived methanol will become key building blocks. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-47 Alkanes and Cycloalkanes End Chapter 3 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 3-48