Structure of hydrocarbons 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

Hydrocarbons

Structure 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

Constitutional Isomerism Constitutional isomers: compounds with the same molecular formula but a different connectivity of their atoms 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 World population is about 6,000,000,000

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

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 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 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 Line-angle drawings each line represents a C-C bond each vertex and line ending represents a C

Cycloalkanes Example: name these cycloalkanes

IUPAC - General Nature of Carbon-Carbon Bonds in the Parent Chain 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

IUPAC - General prop-en-e = propene eth-an-ol = ethanol but-an-one = butanone but-an-al = butanal pent-an-oic acid = pentanoic acid cyclohex-an-ol = cyclohexanol eth-yn-e = ethyne eth-an-amine = ethanamine

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)

Isomers relationships among isomers

Cis,Trans Isomers 1,2-Dimethylcyclopentane

Cis,Trans Isomerism 1,4-Dimethylcyclohexane

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

Cis,Trans Isomerism cis-1,4-Dimethylcyclohexane

Physical Properties Constitutional isomers have different physical properties

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 C O 2 + 9 H 2 O

Heat of Combustion strain in cycloalkane rings as determined by heats of combustion

Sources of Alkanes Natural gas Petroleum Coal 90-95% methane 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

Unsaturated Hydrocarbons Unsaturated hydrocarbon: contains one or more carbon-carbon double or triple bonds Alkene: contains a carbon-carbon double bond and has the general formula CnH2n

Unsaturated Hydrocarbons Alkyne: contains a carbon-carbon triple bond and has the general formula CnH2n-2

Unsaturated Hydrocarbons Arenes: benzene and its derivatives

Benzene & Phenyl Group the phenyl group is not reactive under any of the conditions we describe for alkynes

Structure of Alkenes The two carbon atoms of a double bond and the four atoms attached to them lie in a plane, with bond angles of approximately 120° According to the orbital overlap model, a double bond consists of one sigma bond formed by overlap of sp2 hybrid orbitals one pi bond formed by overlap of parallel 2p orbitals

Structure of Alkenes Length of C-C bonds: single > double > triple Strength of C-C bonds:triple > double > single

Cis-Trans Isomerism Because of restricted rotation about a C-C double bond, groups on adjacent carbons are either cis or trans to each other

Nomenclature - Alkenes Use the infix -en- to show the presence of a carbon-carbon double bond Number the parent chain to give the 1st carbon of the double bond the lower number Follow IUPAC rules for numbering and naming substituents For a cycloalkene, the double bond must be numbered 1,2

Nomenclature - Alkenes

Nomenclature - Alkenes

Nomenclature - Alkenes Some alkenes, particularly low-molecular-weight ones, are known almost exclusively by their common names

Nomenclature - Alkynes IUPAC: use the infix -yn- to show the presence of a carbon-carbon triple bond

Nomenclature - Alkynes Common names: prefix the substituents on the triple bond to the name “acetylene”

Configuration - cis, trans The cis-trans system: configuration is determined by the orientation of atoms of the main chain

Configuration - E,Z The E,Z system uses the priority rules of the R,S system to assign to the groups on each carbon of a carbon-carbon double bond 1. Each atom bonded to the C-C double bond is assigned a priority 2. If groups of higher priority are on the same side, configuration is Z (German, zusammen) 3. If groups of higher priority are on opposite sides, configuration is E (German, entgegen)

Configuration - E,Z Example: name each alkene and specify its configuration by the E,Z system

Physical Properties Alkenes are nonpolar compounds The only attractive forces between their molecules are dispersion forces The physical properties of alkenes are similar to those of alkanes

Terpenes Terpene: a compound whose carbon skeleton can be divided into two or more units identical with the carbon skeleton of isoprene

Terpenes Myrcene, C10H16, a component of bayberry wax and oils of bay and verbena Menthol, from peppermint Camphor, from the camphor tree

Vitamin A (Retinol)

Benzene - Resonance Model The concepts of hybridization of atomic orbitals and the theory of resonance, developed in the 1930s, provided the first adequate description of benzene’s structure and reactivity the carbon skeleton is a regular hexagon all C-C-C and H-C-C bond angles 120°

Benzene - The Resonance Model The pi system of benzene (a) the carbon framework with the six 2p orbitals (b) overlap of the parallel 2p orbitals forms one torus above the plane of the ring and another below it this orbital represents the lowest-lying pi-bonding molecular orbital

Benzene - Resonance We often represent benzene as a hybrid of two equivalent Kekulé structures each makes an equal contribution to the hybrid and thus the C-C bonds are neither double nor single, but something in between

Benzene - Resonance Resonance energy: the difference in energy between a resonance hybrid and the most stable of its hypothetical contributing structures in which electrons are localized on particular atoms and in particular bonds one way to estimate the resonance energy of benzene is to compare the heats of hydrogenation of benzene and cyclohexene

Benzene

Concept of Aromaticity The underlying criteria for aromaticity were recognized in the early 1930s by Erich Hückel, based on molecular orbital (MO) calculations To be aromatic, a compound must be cyclic have one p orbital on each atom of the ring be planar or nearly planar so that there is continuous or nearly continuous overlap of all p orbitals of the ring have a closed loop of (4n + 2) pi electrons in the cyclic arrangement of p orbitals

Heterocyclic Aromatics Heterocyclic compound: a compound that contains more than one kind of atom in a ring in organic chemistry, the term refers to a ring with one or more atoms are other than carbon Pyridine and pyrimidine are heterocyclic analogs of benzene; each is aromatic.

Nomenclature of aromatic compounds Monosubstituted alkylbenzenes are named as derivatives of benzene many common names are retained T oluene E thylbenzene C umene S tyrene O H N H 2 C H O C O H O C H 3 Phenol Aniline Benzaldehyde Benzoic acid Anisole

Nomenclature Benzyl and phenyl groups C H C H - Benzene P 3 C H 2 - Benzene P henyl group, Ph- Toluene B enzyl group, Bn- O O H 3 C O P h 1-Phenyl-1pentanone 4-(3-Methoxyphenyl)- 2-butanone (Z)-2-Phenyl- 2-butene

Disubstituted Benzenes Locate two groups by numbers or by the locators ortho (1,2-), meta (1,3-), and para (1,4-) where one group imparts a special name, name the compound as a derivative of that molecule

Polysubstituted Derivatives if one group imparts a special name, name the molecule as a derivative of that compound if no group imparts a special name, list them in alphabetical order, giving them the lowest set of numbers C H 3 O H N O 2 1 1 N O 2 B r B r 4 2 6 2 2 B r 4 4 1 C l B r C H 2 3 4-Chloro-2-nitro- toluene 2,4,6-Tribromo- phenol 2-Bromo-1-ethyl-4- nitrobenzene

Phenols The functional group of a phenol is an -OH group bonded to a benzene ring O H O H O H O H O H C H 3 O H Phenol 3-Methylphenol ( m- Cresol) 1,2-Benzenediol (Catechol) 1,4-Benzenediol (Hydroquinone)

Phenols hexylresorcinol is a mild antiseptic and disinfectant eugenol is used as a dental antiseptic and analgesic urushiol is the main component of the oil of poison ivy

Quinones Important chemical property of quinones is that they are readily reduced to hydroquinones