Unsaturated Hydrocarbons Chapter 20 Larry Emme Chemeketa Community College

Unsaturated hydrocarbons enhance our lives in many ways: Polyethylene plastic bags and bottles Polystyrene Styrofoam cups Plastic wraps Essential oils in plants contain multiple bonds between carbon atoms. Cosmetics, medicines, flavorings, perfumes 5. Hydrocarbons also form rings of carbon atoms (aromatics) Detergents, insecticides, and dyes

Fragrant Alkenes

Bonding in Unsaturated Hydrocarbons

The unsaturated hydrocarbons consist of three families of homologous compounds that contain multiple bonds between carbon atoms. Alkenes contain carbon-carbon double bonds. Alkynes contain carbon-carbon triple bonds. Aromatic compounds contain benzene rings.

The four orbitals available for bonding in alkenes are three sp2 orbitals and one p orbital. Schematic hybridization of 2s22px12py1 orbitals of carbon to form three sp2 electron orbitals and one p electron orbital

p (a) A single sp2 electron orbital and (b) a side view of three sp2 orbitals all lying in the same plane with a p orbital perpendicular to the three sp2 orbitals.

The carbon-carbon pi () bond is much weaker and, as a consequence, much more reactive than the carbon-carbon sigma () bond. p p p p

The formation of a triple bond between carbon atoms, as in acetylene, CHCH, may be visualized as shown below. These pi bond electrons are not as tightly held by the carbon nuclei as the sigma bond electrons. Acetylene, consequently, is a very reactive substance. sideways overlap of p orbitals forms two  bonds at right angles  bond formed by sp-sp overlap  bond formed by s-sp overlap Acetylene results

Nomenclature of Alkenes

The general formula for alkenes is: CnH2n

IUPAC Rules for Naming Alkenes 1. Select the longest continuous carbon-carbon chain that contains the double bond. 2. Name this parent compound as you would an alkane, but change the –ane ending to –ene. CH3CH2CH3 propane CH3CH=CH2 propene

IUPAC Rules for Naming Alkenes 3. Number the carbon chain of the parent compound starting with the end nearer to the double bond. Use the smaller of the two numbers on the double-bonded carbon atoms to indicate the position of the double bond. Place this number in front of the alkene name. 1 2 3 4 CH3CH=CHCH3 2- butene CH3CH2CH2CH=CH2 1-pentene 5 4 3 2 1

IUPAC Rules for Naming Alkenes 4. Branch chains and other groups are treated as in naming alkanes, by numbering and assigning them to the carbon atom to which they are bonded.

IUPAC Rules for Naming Alkenes 4. Branch chains and other groups are treated as in naming alkanes, by numbering and assigning them to the carbon atom to which they are bonded. Better would be n-propyl 3-n-propyl-1-hexene

How would we write the structural formula for 4-methyl-2-pentene? The name indicates: Five carbons in the longest chain A double bond between carbons 2 and 3 A methyl group on carbon 4

How would we write the structural formula for 4-methyl-2-pentene? The name indicates: Five carbons in the longest chain A double bond between carbons 2 and 3 A methyl group on carbon 4 CH3 CH3CHCHCHCH3 1 2 3 4 5

Condensed & Line formulas 1 2 3 4 5 CH3 CH3CHCHCHCH3

Write a structural formula for: 7-methyl-2-octene Octene indicates an 8-carbon chain The chain contains a C=C between carbons 2 and 3. There is a –CH3 group on carbon 7

Write a structural formula for: 7-methyl-2-octene Octene indicates an 8-carbon chain The chain contains a C=C between carbons 2 and 3. There is a –CH3 group on carbon 7

Condensed & Line formulas CH3CHCHCH2CH2CH2CHCH3 CH3

Name this compound:

Name this compound: Longest chain containing C=C is 5 carbons ethyl 5 3 4 2 1 Longest chain containing C=C is 5 carbons 2-ethyl-1-pentene

Condensed & Line formulas CH3CH2 CCH2CH2CH3 CH2

Geometric Isomerism in Alkenes

Geometric Isomerism in Alkenes Compounds containing a carbon-carbon double bond (pi bond) have restricted rotation about that double bond. This restricted rotation in a molecule gives rise to a type of isomerism known as geometric isomerism. Isomers that differ from each other only in the geometry of their molecules and not in the order of their atoms are known as geometric isomers. They are also called cis-trans isomers.

Trans is a Latin noun or prefix, meaning “across”, “beyond” or “on the opposite side”. Cis is a Latin prefix, meaning “on the same side [as]” , “on this side [of]”, or “near side [of]”.

Geometric Isomers in Alkenes

An alkene shows cis-trans isomerism when each carbon atom of the double bond has two different kinds of groups attached to it.

An alkene does not show cis-trans isomerism if even one carbon of the double bond has two identical groups attached to it.

Draw a structure for cis-5-chloro-2-hexene The compound contains 6 carbons with a C=C between carbons 2 and 3, and a Cl atom on carbon 5.

Draw a structure for cis-5-chloro-2-hexene The compound contains 6 carbons with a C=C between carbons 2 and 3, and a Cl atom on carbon 5. 6 4 5 1 2 3

Condensed cis-5-chloro-2-hexene 6 4 5 1 2 3 CH3CCCH2CH(Cl)CH3 H H

Is the compound below the cis or trans isomer?

Is the compound below the cis or trans isomer? 1 2 3 4 5 trans-3-methyl-2-pentene

Condensed trans-3-methyl-2-pentene 1 2 3 4 5 CH3 CH3CCCH2CH3 H

Cycloalkenes

Cycloalkenes As the name implies, cycloalkenes are cyclic compounds that contain a C=C in the ring. The carbons of the double bond are assigned numbers 1 and 2.

Cycloalkenes

Preparation and Physical Properties of Alkenes

Preparation of Alkenes Cracking Dehydration of Alcohols

Cracking Cracking, or pyrolysis, is the process in which saturated hydrocarbons are heated to very high temperatures in the presence of a catalyst (usually silica-alumina): Alkane (CnH2n+2)  Mixture of alkenes + Alkanes + H2 (g) 2CH3CH2CH3  CH3CH=CH2 + CH2=CH2 + CH4 + H2 Heat catalyst ~500°C

Dehydration of Alcohols Dehydration involves the elimination of a molecule of water from a reactant molecule.

Physical Properties of Alkenes Very similar to the corresponding alkanes. Name Molecular formula CnH2n Condensed formula Boiling pt (°C) Melting pt (°C) Ethene C2H4 CH2CH2 104 169 Propene C3H6 CH3CHCH2 48 185 1-Butene C4H8 CH3CH2CHCH2 6 2-Methylpropene (CH3)2CCH2 7 14 1-Pentene C5H10 CH3(CH2)2CHCH2 30 138

Chemical Properties of Alkenes

Addition Reactions of Alkenes Addition at the C=C bond is the most common reaction of alkenes. H2 Br2 and Cl2 HBr, HCl H2O

Addition of H2 Hydrogenation

Addition of X2 Bromination (Br2) or Chlorination (Cl2) Bromine changes from a red-orange (flask on the left) to colorless when added to an alkene as shown in the flask on the right.

Addition of HX Hydrobromination (HBr) or Hydrochlorination (HCl)

Addition of H2O HOH

Addition of HX to an Unsymmetrical Alkene Why???? Them that has, gets!

Markovnikov’s Rule When an unsymmetrical molecule such as HX (HCl) adds to a carbon-carbon double bond, the hydrogen from HX goes to the carbon atom that has the greater number of hydrogen atoms. Vladimir Markovnikov

Markovnikov’s Rule This reaction proceeds via the formation of the most stable carbocation intermediate (2°).

Write formulas for the organic products formed when 2-methyl-1-butene reacts with: H2, Pt/25°C Cl2 HCl H20, H+

2-methyl-1-butene + H2, Pt/25 °C 2-methylbutane

2-methyl-1-butene + Cl2 1,2-dichloro-2-methylbutane

2-methyl-1-butene + HCl 2-chloro-2-methylbutane

2-methyl-1-butene + H2O 2-hydroxy-2-methylbutane

Oxidation

Oxidation at the C=C Bond Baeyer Test + –

Alkynes: Nomenclature and Preparation

IUPAC Rules for Naming Alkynes The rules for naming alkynes are the same as those for alkenes, but the ending –yne is used to indicate the presence of a triple bond.  CH3CH2CCH 1-butyne

Nomenclature of Common Alkynes Molecular formula CnH2n-2 Condensed formula IUPAC Name C2H2 HCCH or CHCH Ethyne or acetylene (common) C3H4 CH3CCH Propyne C4H6 CH3CH2CCH 1-Butyne CH3CCCH3 2-Butyne     

Physical and Chemical Properties of Alkynes

Physical Properties of Alkynes Acetylene is a colorless gas with little odor when pure. Acetylene is insoluble in water and is a gas at normal temperature and pressure.

Chemical Properties of Alkynes Alkynes undergo addition reactions rather similar to those of alkenes. Cl2 and Br2 HCl and HBr Positive reaction with Baeyer’s test.

Bromination of Acetylene HCCH + Br2  CHBr=CHBr HCCH + 2 Br2  CHBr2-CHBr2

Tests for Unsaturation alkene (alkyne) alkene (alkyne) alkane alkane Bromine Potassium permanganate (Baeyer’s test)

HCl Addition to Unsymmetrical Alkynes This addition follows Markovnikov’s rule: CH3CCH + HCl  CH3CCl=CH2 CH3CCH + 2 HCl  CH3CCl2-CH3

Aromatic Hydrocarbons: Structure

Benzene, or benzol, is an compound with the molecular formula C6H6 Benzene, or benzol, is an compound with the molecular formula C6H6. It is sometimes abbreviated Ph–H. Michael Faraday (1791 –1867 ) first isolated and identified benzene in 1825 from the oily residue derived from the production of illuminating gas, giving it the name bicarburet of hydrogen. 1820 The empirical formula for benzene was long known, but its highly polyunsaturated structure, with just one hydrogen atom for each carbon atom, was challenging to determine. Several in 1861 suggested possible structures that contained multiple double bonds or multiple rings, but the study of aromatic compounds was in its very early years, and too little evidence was then available to help chemists decide on any particular structure.

Friedrich Auguste Kekulé 1829-1896

Friedrich August von Kekule had a dream of whirling snakes, of the structure of benzene - the organic chemical compound made up of a ring of carbon atoms. He reported the dream in the following words many years after it took place, in a speech at a dinner commemorating his discovery. I turned my chair to the fire (after having worked on the problem for some time) and dozed. Again the atoms were gamboling before my eyes. This time the smaller groups kept modestly to the background. My mental eye, rendered more acute by repeated vision of this kind, could not distinguish larger structures, of manifold conformation; long rows, sometimes more closely fitted together; all twining and twisting in snakelike motion. But look! What was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes. As if by a flash of lighting I awoke... Let us learn to dream, gentlemen.

Aromatic Compounds Benzene is an aromatic compound. a ring of 6 C atoms and 6 H atoms. a flat ring structure drawn with three double bonds. represented by two structures because the electrons are shared among all the C atoms. 74 74

Bonding in Benzene The electrons are not attached to particular carbon atoms, but are delocalized and associated with the entire molecule. This electronic structure imparts unusual stability to benzene and is responsible for many of the characteristic properties of aromatic compounds.

Bonding in Benzene (a) sp2-sp2 orbital overlap to form the carbon ring structure.

Bonding in Benzene (b) carbon-hydrogen bonds formed by sp2-s orbital overlap and overlapping p orbitals.

Bonding in Benzene (c) pi electron clouds above and below the plane of the carbon ring.

Naming Aromatic Compounds

Naming Substituted Benzene Compounds A substituted benzene is derived by replacing one or more hydrogen atoms of benzene by another atom or group of atoms. Monosubstituted benzene has the formula C6H5G, where G is the group replacing a hydrogen atom.

Monosubstituted Benzenes Some monosubstituted benzenes are named by adding the name of the substituent group as a prefix to the word benzene.

Variations of writing side-groups   

Certain monosubstituted benzenes have special names.

The word phenyl represents the C6H5- group. It is used to name benzene derivatives that would otherwise be difficult to name.

Disubstituted Benzenes The prefixes ortho-, meta-, and para- (abbreviated o-, m-, and p-) are used to name disubstituted benzenes. Greek: right, straight Greek: after, with Greek: beyond, beside

Dichlorobenzenes, C6H4Cl2 The three isomers of dichlorobenzene have different physical properties.

Disubstituted Benzenes When the two substituents are different and neither is part of a compound with a special name, the names of the two substituents are given in alphabetical order, followed by the word benzene.

Isomers of Dimethyl Benzene The dimethylbenzenes have the special name xylene.

Isomers of Methylphenol The methylphenols have the special name cresol. OH ortho-cresol meta-cresol para-cresol

Disubstituted Benzenes When one of the substituents corresponds to a monosubstituted benzene that has a special name, the disubstituted compound is named as a derivative of that parent compound.

Polysubstituted Benzenes When there are more than two substituents on a benzene ring, the carbon atoms in the ring are numbered starting at one of the substituted groups. Numbering must be done in the direction that gives the lowest possible numbers to the substituent groups.

Polysubstituted Benzenes

Polycyclic Aromatic Compounds

Polycyclic Aromatic Hydrocarbons PAHs mothballs dyes carcinogen Product of combustion of coal tar, tobacco smoke, barbecued meats. Potent carcinogen

Sources and Physical Properties of Aromatic Hydrocarbons

Sources of Aromatic Hydrocarbons The aromatic hydrocarbons, such as benzene, toluene, xylene, naphthalene, and anthracene, were first obtained in significant quantities from coal tar. Coal  Coke + Coal gas + Coal tar Because of the great demand for aromatic hydrocarbons, processes were devised to obtain them from petroleum. 

Properties of Aromatic Hydrocarbons Aromatic hydrocarbons are essentially nonpolar substances, insoluble in water but soluble in many organic solvents. They are liquids or solids and usually have densities less than that of water. Aromatic hydrocarbons burn readily, usually with smoky (sooty) yellow flames as a result of incomplete carbon combustion.

Chemical Properties of Aromatic Hydrocarbons

Substitution Reactions of Aromatic Hydrocarbons Halogenation net addition of -Br or -Cl Nitration net addition of –NO2 Alkylation net addition of –R (alkyl group)

Halogenation of Benzene When benzene reacts with chlorine or bromine in the presence of a catalyst such as iron (III) chloride or iron (III) bromide, a Cl or Br atom replaces an H atom to form the products.

Nitration of Benzene When benzene reacts with a mixture of concentrated nitric acid and concentrated sulfuric acid at about 50C, nitrobenzene is formed.

Alkylation of Benzene Alkylation of benzene is known as the Friedel-Crafts reaction. The alkyl group from an alkyl halide (RX), in the presence of AlCl3 catalyst, substitutes for an H atom on the benzene ring.

Mr. Ortho Paranoid

Ms. Molly Meta

Ortho/Para Directors Groups present on the benzene ring as substitution reactions are occurring, may direct oncoming groups to the ortho or para positions. These groups usually have unshared electron pairs next to the ring. Examples include: –OH, –NH2, –OCH3, – CH3, –Br, and –Cl

Examples of Ring Directors ortho- para-

Meta Directors Groups present on the benzene ring as substitution reactions are occurring, may direct oncoming groups to the meta position by deactivating the ortho and para positions. These groups have a positive charge next to the ring. Examples include: –NO2 and –CN

Examples of Ring Directors meta – only

Side-Chain Oxidation Carbon chains attached to an aromatic ring are fairly easy to oxidize.

The End