Alkanes and Cycloalkanes Unit 4

Structure Hydrocarbon: a compound composed only of carbon and hydrogen Saturated hydrocarbon: a hydrocarbon containing only single bonds Carbon can form four single covalent bonds to other atoms Alkane: a saturated hydrocarbon whose carbons are arranged in an open chain General formula: Cn + H2n + 2 Aliphatic hydrocarbon: another name for an alkane

Hydrocarbons H - C y d r o c a b n s A l k e Aromatic O i g m u t p z x N h B S U

Structure OF ALKANES Shape tetrahedral about carbon all bond angles are approximately 109.5°

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

Isomers relationships among isomers

Constitutional Isomerism Constitutional (structural) isomers: compounds with the same molecular formula but a different connectivity of their atoms Exhibit different properties: 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 The number of isomers increases rapidly as the number of carbon atoms increases

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

Alkyl and Aryl Groups Chapter 19

Nomenclature - IUPAC Parent name: the longest carbon chain When a hydrogen is removed from an alkane, an alkyl group results Substituent: a group bonded to the parent chain alkyl group: a substituent derived by removal of a hydrogen from an alkane; When we remove the hydrogen atom, the name -ane suffix is changed to -yl (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 five- and six-membered rings are the most common and most stable 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 Monocyclic Compounds Cycloalkanes with only one ring Attach the prefix cyclo-

Substituted Cycloalkanes

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

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)

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

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

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—the best way to judge the stability of the different conformations of a molecule

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

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

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

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

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

Bicycloalkanes Norbornane drawn from three different perspectives

Bicycloalkanes Adamantane

Geometric Isomers 1 Same side: cis- 2 2 Opposite side: trans- 1 cis-1,2-dimethylcyclohexane 2 2 Opposite side: trans- 1 trans-1-ethyl-2-methylcyclohexane

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 Cycloalkanes are non-polar, relatively inert

Physical Properties Constitutional isomers have different physical properties

Effect of Branching on Boiling Point Branching lowers b.p. because of decreased surface contact between molecules. The long-chain isomer (n-pentane) has the greatest surface area and the highest boiling point. As the amount of chain branching increases, the molecule becomes more spherical and its surface area decreases. The most highly branched isomer (neopentane) has the smallest surface area and the lowest boiling point.

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 dioxide and water

Heat of Combustion Heat of combustion for constitutional isomers

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

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