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Alkanes can also form cyclic structures
Cyclopropane Cyclobutane Cyclopentane Cyclohexane General formula for cycloalkanes: CnH2n Can be conveniently represented using line segment formulae
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Cycloalkane nomenclature can be extended to include substitution
Note: Cycloalkane nomenclature can be extended to include substitution Methylcyclohexane 1,3-Dimethylcyclohexane
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Only one cycloalkane has a planar structure: cyclopropane
All others have non-planar structure Ideal tetrahedral angle is 109.5o sp3 hybridised carbons with bond angles very different to 109.5o will be less stable (higher in energy) Bond angle approaching 60o Cyclopropane Cyclopropane is said to suffer from angle-strain All C-H bonds in cyclopropane are eclipsed
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Cyclopentane has almost zero angle-strain
To relieve torsional strain due to eclipsed C-H bonds, cyclopentane relaxes into a non-planar structure One CH2 group out of the plane of the ring
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Cyclohexane A planar structure would have internal bond angles of 120o and eclipsed C-H bonds Actual structure relaxes into a chair conformation This reduces the bond angle to 109o Geometry about each Carbon very close to tetrahedral ideal Angle strain ~ zero
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All C-H bonds staggered, i.e. torsional strain ~ zero
Newman projection along any C-C bond The chair conformation contains two different hydrogen environments 6 Equatorial Hydrogens 6 Axial Hydrogens
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Exists in trace quantities
At temperatures below 230 K (-43C): can observe that two different types of hydrogen environment are present on cyclohexane Above this temperature, observe only one hydrogen environment Reason: cyclohexane molecules are not static above 230 K i.e. exist in different conformations Undergo ring inversion Boat conformation Exists in trace quantities Note: hydrogens axial in one chair conformation equatorial in the other
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Ball-and-stick model of boat cyclohexane
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What if one of the cyclohexane hydrogens were replaced by a methyl group?
Methylcyclohexane The two chair conformations are no longer equivalent One has the methyl group in an axial position; one in an equatorial position
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These interconvert by ring inversion (exist in equilibrium)
[Inversion proceeds through boat conformations which exist in trace amounts] Can simplify diagram by omitting the C-H bonds Methyl equatorial Methyl axial
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Sources of alkanes Lower Mol. Mt. (~ < 5 Carbons): natural gas Larger Mol. Wt.: petroluem of crude oil Crude oil: complex mixture of hydrocarbons Separated into fractions based on boiling point ranges Boiling point related to molecular weight, i.e to number of carbons < 5 Carbons: gases at room temperature 5 Carbons < ~18 Carbons: liquids at room temperature > 18 Carbons: solids at room temperature
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Increasing molecular size results in increasing tendency to form condensed phases
Associated with weak intermolecular interactions between alkane molecules London dispersion forces: weak electrostatic attractions between induced dipoles, i.e. are… Van der Waals’ forces between electrons of one molecule and nuclei of another Extent of attraction increases with increasing molecular size Weak interactions compared to hydrogen bonding or ionic bonding
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Solubility of alkanes ‘Like dissolves like’: alkanes soluble in other alkanes, e.g petroleum [Soluble: single liquid phase results upon mixing] Alkanes insoluble in water, i.e are hydrophobic Mixtures with water separate into two liquid phases: aqueous and hydrocarbon
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2 C4H10 + 13 O2 → 8 CO2 + 10 H2O CO2 → Urea DH = - 2877 kJ mol-1
Reactions of alkanes Relatively inert; contain only stable C-C and C-H bonds Some important reactions: 1. Combustion, e.g. 2 C4H O2 → 8 CO H2O DH = kJ mol-1 i.e. exothermic 2. Steam reforming CH4 + H2O → 3H2 + CO N2 ↓ ↓ NH3 CO2 → Urea +
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3. Reaction with halogens
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4. Catalytic cracking Fragmentation of alkanes into smaller molecules, e.g: The products of these reactions are a new type of hydrocarbon They are said to be ‘unsaturated’ compared to alkanes i.e., have fewer Hydrogens per Carbon than alkanes, which are said to be ‘saturated’
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Alkenes: contain Carbon-Carbon double bonds
Unsaturated hydrocarbons contain Carbon-Carbon multiple bonds Classes of unsaturated hydrocarbons are defined by the types of Carbon-Carbon multiple bonds they contain Alkenes: contain Carbon-Carbon double bonds Carbon-Carbon double bond Alkynes: contain Carbon-Carbon triple bonds Carbon-Carbon triple bond Carbon valency of four maintained in alkenes and alkynes
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Alkenes Older name: Olefins
Characterised by presence of Carbon-Carbon double bonds General structural formula Where ‘R’ = Hydrogen or alkyl group Two Carbons and all four ‘R’ groups are lying on the same plane Bond angles about each Carbon ~ 120o
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Three sp2 hybridised orbitals can be arrayed to give trigonal geometry
The remaining 2pz orbital is orthogonal to the three sp2 orbitals View along z axis View along xy plane
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s bond formation results from overlap of two sp2 hybridised orbitals
[A s-antibonding orbital is also formed, but this is not occupied by electrons] Overlap of the pz orbitals results in formation of a p bond [A p-antibonding orbital is also formed, but this is not occupied by electrons] p
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p s p orbital: has a nodal plane on which lies on the bond axis
p electron density lies above and below the plane containing the two Carbons and four ‘R’ groups View along the Carbon-Carbon bond Note: constitutes one p molecular orbital i.e. constitutes one p bond when occupied p Carbon-Carbon double bond: One s bond; One p bond s Both occupied by two electrons
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Rotation about a Carbon-Carbon double bond requires opening up of the p bond
Requires large input of energy (~ 268 kJ mol-1) Hence, rotation about C=C bonds does not occur at room temperature Consequently, a new form of isomerism becomes possible for alkenes Consider an alkene with one Hydrogen and one alkyl group ‘R’ bonded to each Carbon Two structures are possible or
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Cis isomer Trans isomer
This form of isomerism is known as Cis-Trans isomerism [older term: geometrical isomerism] The cis isomer is that with like groups on the same side of the C=C The trans isomer is that with like groups on opposite sides of the C=C Cis isomer Trans isomer
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First two members of the alkene series:
Ethene (Ethylene) Propene (Propylene) Note: Nomenclature: Prefix indicates number of carbons (‘eth…’ = 2C; ‘prop…’ = 3C; etc.) Suffix ‘…ene’ indicates presence of C=C
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Could have C=C between C1 and C2
or between C2 and C3 Butene 1-Butene 2-Butene Note: 1. 1-Butene and 2-butene are structural isomers 2. 3. Number indicates starting point of the C=C, i.e. number through the C=C
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4. Cis-Trans isomerism is possible for 2-butene
There are two isomeric 2-butenes Trans-2-butene b.p. 3.7oC m.p. -139oC Cis-2-butene b.p. 0.3oC m.p. -106oC
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Some other alkenes 4-Methyl-2-pentene 2-Methyl-1-butene 1,3-Pentadiene Cis-3-heptene Trans-2-decene
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Can have cycloalkenes Cyclohexene Cyclopentene 3-Methylcyclopentene Note: 1,4-Cyclohexadiene
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Lycopene molecular structure
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p electrons in alkenes are available to become involved in bond formation processes
Essential processes in the synthesis of new molecules: formation of new covalent bonds Covalent bonds: pairs of electrons shared between nuclei (atoms) In the synthesis of organic molecules, a major strategy for forming new covalent bonds is: donation of an electron pair by one molecular species… …to form a covalent bond with another, electron deficient molecular species Electron pair donating species are known as nucleophiles Electron pair accepting species are known as electrophiles Reaction of a nucleophile with an electrophile results in the formation of a new covalent bond
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Alkene hydrogenation Addition of hydrogen (H2) across a C=C General reaction Alkene p bond is lost, and two new C-H s bonds formed Alkene converted to alkane No reaction in absence of catalyst Typical catalysts: Palladium (Pd), Platinum (Pt), Nickel (Ni), Rhodium (Rh) or other metals Catalysts usually supported on materials such as charcoal E.g. Pd/C “Palladium on Carbon”
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Examples 1-Hexene Hexane Hexane 1,3-Hexadiene 2-Methyl-1-butene
2-Methylbutane
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Reaction occurs at the catalyst surface
H2 molecules adsorbed onto catalyst surface Both Hydrogens added to same face of C=C 1,2-Dimethylcyclohexene Cis-1,2-dimethylcyclohexane Both Hydrogens added to the same face of the cyclohexene C=C [Cis/Trans naming system can be extended to cyclic systems]
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C=C p bond lost; new C-H and C-X s bonds formed
Addition of HX to alkenes General reaction X = Cl, Br, I C=C p bond lost; new C-H and C-X s bonds formed e.g: 2-Chloropropane (only product) 1-Chloropropane (not formed) Propene To explain this, need to consider the reaction mechanism
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Reaction mechanism: detailed sequence of bond breaking and bond formation in going from reactants to products Addition of HX to alkenes: reaction involves two steps 1st Step: Addition of proton (H+) 2nd Step: Addition of halide (X-) 1st Step Alkene p electrons attack proton New C-H s bond results Remaining Carbon short 1 electron Carbon positively charged
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Addition of H+ to the alkene p bond forms a new C-H s bond and a carbocation intermediate
[or carbonium ion] 2nd Step New C-X s bond results Halide ion attacks electron deficient carbon
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Reaction of HCl with CH3-CH=CH2
1st Step: addition of H+ to form a carbocation intermediate Two possible modes of addition or I.e. two possible carbocation intermediates
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Classification of carbocations
Primary (1o) Carbocation Secondary (2o) Carbocation Tertiary (3o) Carbocation 2o Carbocation 1o Carbocation
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Most stable 3o > 2o > 1o Least stable
The relative order of stability for carbocations is: Most stable 3o > 2o > 1o Least stable This is because carbocations can draw electron density along s bonds; known as an inductive effect This effect is significant for alkyl substituents, but weak for Hydrogens Least stabilised Most stabilised
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Addition of HCl to CH3-CH=CH2 proceeds so as to give the more stable of the two possible carbocation intermediates, i.e: Not formed Addition of chloride then gives 2-chloropropane exclusively Cl- Additions of HX to alkenes which follow this pattern are said to obey Markovnikov’s rule “Reaction proceeds via the more stable possible carbocation intermediate”
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Other examples not 2-Methylpropene 2-Bromo-2-methyl- propane
1-Methylcyclohexene 1-Chloro-1-methyl- cyclohexane 1-Chloro-2-methyl- cyclohexane 2-Butene 2-Chlorobutane (Symmetrical alkene) Same structure
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Addition of water to alkenes Follows same pattern as addition of HX
Acid catalysis required Propene 2-Hydroxypropane (2-Propanol) Mechanism: 1. Protonation of C=C so as to give the more stable carbocation intermediate
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2. Attack on the carbocation by water acting as a nucleophile
3. Loss of proton to give the product and regenerate the catalyst
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Acid catalysed addition of water often difficult to control
A Mercury (II) mediated version often used - oxymercuration 1-Methylcyclopentene 1-Hydroxy-1-methyl- cyclopentane Gives exclusively Markovnikov addition Hydroboration 1-Methylcyclopentene 1-Hydroxy-2-methyl- cyclopentane Gives exclusively anti-Markovnikov addition Mechanisms of these reactions beyond the scope of this module
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Alkene p bond lost; two new C-OH s bonds formed
Alkene hydroxylation Alkene p bond lost; two new C-OH s bonds formed Alkene epoxidation Epoxides Alkene p bond lost; two new C-O s bonds are formed to the same Oxygen
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Examples Propene Propane-1,2-diol 1,2-Epoxypropane Cyclopentene
1,2-Epoxycyclopentane Cis-1,2-cyclopentanediol
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Ozonolysis of alkenes Ozone (O3): strong oxidising agent Adds to C=C with loss of both the p and s bonds Products formed are known as ozonides Ozonide Ozonides usually not isolated, but further reacted with reducing agents Formation of two molecules each containing C=O (Carbonyl) groups
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Overall process: Examples 1-Butene Aldehydes Ketone
2,3-Dimethyl-2-butene
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Addition of bromine (Br2) to alkenes
General reaction Alkene p bond lost; two new C-Br s bonds formed Stereospecific reaction observed with cycloalkenes Cyclopentene Trans-1,2-dibromo- cyclopentane (no cis-isomer)
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Chlorine also adds to alkene C=C bonds
1,2-Dichlorobutane 1-Butene
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All Carbons and Hydrogens equivalent
Benzene Molecular formula C6H6 All Carbons and Hydrogens equivalent Kekulé structure (1865) = However, does not behave like a typical alkene Less reactive than typical alkenes Only reacts with bromine in presence of a catalyst A substitution rather than an addition reaction occurs not
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Styrene
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Arrangement of 6 p electrons in a closed cyclic p systems is especially stable
Said to possess aromaticity Aromatic systems very common (e.g. benzene and its derivatives) Representing the p system in benzene Represents p system well Of limited use in describing reactivity Better to use a combination of Kekulé structures
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These are NOT independent species existing in equilibrium
The p electrons in benzene are said to be resonance delocalised over the entire ring system Resonance delocalisation is generally energetically favourable Resonance delocalisation of 6 p electrons in a closed ring system is especially favourable: aromaticity
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Alkynes Older name: Acetylenes
Characterised by the presence of Carbon-Carbon triple bonds General structure of alkynes Groups R, C, C and R are co-linear Neither sp3 nor sp2 hybridised Carbon consistent with this geometry
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Two sp hybridised orbitals can be arrayed to give linear geometry
Two remaining 2p orbitals are mutually orthogonal and orthogonal to the two sp hybridised orbitals [If the two sp orbitals lies along the z axis, 2px lies along the x axis and 2py along the y axis]
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The s bond lies along the C-C bond axis
C≡C consists of one s bond and two p bonds The s bond lies along the C-C bond axis The bond axis lies along the intersection of orthogonal planes One p bond lies in each plane, with a node along the bond axis View along the bond axis
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First two members of the series of alkynes
Ethyne (Acetylene) Propyne Nomenclature Prefix indicates number of carbons (‘eth…’, ‘prop…’, etc.) Suffix ‘…yne’ indicates presence of C≡C Butyne Can have C≡C between C1 and C2 or between C2 and C3 1-Butyne 2-Butyne These are structural isomers
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6-Methyl-3-octyne 1-Heptene-6-yne 4-Methyl-7-nonen-1-yne
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Linear geometry of alkynes difficult to accommodate in a cyclic structure
Hence relatively few cycloalkynes Smallest stable cycloalkyne is cyclononyne Cyclononyne
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Hydrogenation of alkynes
Standard hydrogenation conditions completely remove the p bonds Both p bonds lost; four new C-H s bonds formed Heptane 3-Heptyne [Conversion of alkyne to alkane]
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Alkyne Cis-alkene 3-Heptyne Cis-3-heptene
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This gives specifically Trans-alkenes
Alkynes can also be converted into alkenes by reaction with sodium or lithium metal in liquid ammonia [Na, liq. NH3; or Li, liq. NH3] This gives specifically Trans-alkenes 3-Heptyne Trans-3-heptene
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Cis-2-hexene Trans-2-hexene
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Addition of bromine (Br2) to alkynes
Can have addition to one or both alkyne p bonds Alkyne Trans-1,2-dibromo- alkene 1,1,2,2-tetra- bromoalkane 1,1,2,2-Tetrabromoethane Ethyne (Acetylene) Trans-1,2-dibromo- 1-butene 1-Butyne
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Requires catalysis by mercury (II) salts
Hydration of 1-alkynes [Addition of water] Requires catalysis by mercury (II) salts 1-Alkyne Ketones 4-Methyl-1-hexyne Ketone
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