Alkanes Nomenclature Conformational Analysis and an Introduction to

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Alkanes Nomenclature, Conformational Analysis, and an Introduction to Synthesis © E. V. Blackburn, 2005

Alkanes • acyclic hydrocarbons • saturated aliphatic hydrocarbons • paraffins • general formula Cn. H 2 n+2 © E. V. Blackburn, 2005

Sources of methane • product of anaerobic plant decay • major constituent of natural gas (97%) • “firedamp” of coal mines • “marsh gas” © E. V. Blackburn, 2005

Cycloalkanes Single ring cycloalkanes have the general formula Cn. H 2 n thus they have two fewer hydrogen atoms than alkanes. © E. V. Blackburn, 2005

Methane – its structure tetrahedral © E. V. Blackburn, 2005

Methane – its structure “Fischer Structure” “Lewis Structure” © E. V. Blackburn, 2005

Space-filling models depict atoms as spheres and therefore show the volume occupied by atoms and molecules. © E. V. Blackburn, 2005

Ethane - C 2 H 6 1. 10Å sp 3 1. 53Å A structural formula is a Lewis structure which shows the connectivity of its atoms - the order in which atoms are connected. © E. V. Blackburn, 2005

What is ethane’s structure? Or something in between? © E. V. Blackburn, 2005

Conformations are structures that are interconvertible by rotation about single bonds. This is the staggered conformation of ethane: This is an example of a sawhorse formula. © E. V. Blackburn, 2005

Newman projections Look along the C-C bond. The nearest carbon masks the rear carbon but all six bonds to the two carbons are visible. The nearest carbon is represented by the point where three bonds meet. The rear carbon is represented by the circle. © E. V. Blackburn, 2005

Space-filling model of ethane staggered eclipsed © E. V. Blackburn, 2005

Potential energy Stability of conformations © E. V. Blackburn, 2005

Torsional strain Torsional energy is the energy required to rotate the molecule about the C-C bond. The relative instability of the eclipsed conformation is said to be due to torsional strain. © E. V. Blackburn, 2005

Propane - C 3 H 8 energy barrier = 14 k. J/mol © E. V. Blackburn, 2005

Butane - C 4 H 10 compound bp mp solubility C 2 H 5 OH A -12 -159 1320 B 0 C -138 1813 m. L/100 m. L © E. V. Blackburn, 2005

Conformations All conformations are free of torsional strain. © E. V. Blackburn, 2005

Stability of conformations The methyl groups in the gauche conformations are crowded together and steric repulsion results. These conformations are less stable due to steric strain. © E. V. Blackburn, 2005

Stability of conformations anti gauche © E. V. Blackburn, 2005

Potential energy Stability of conformations © E. V. Blackburn, 2005

Nomenclature CH 4 methane C 2 H 6 ethane C 3 H 8 propane C 4 H 10 butane Subsequent alkanes are systematically named using a numeric prefix (Greek) (penta-, hexa-, etc. ) and the suffix ane. © E. V. Blackburn, 2005

Nomenclature CH 4 methane C 7 H 16 heptane C 2 H 6 ethane C 8 H 18 octane C 3 H 8 propane C 9 H 20 nonane C 4 H 10 butane C 10 H 22 decane C 5 H 12 pentane C 11 H 24 undecane C 6 H 14 hexane C 12 H 26 dodecane C 13 H 28 tridecane C 14 H 30 tetradecane C 20 H 42 icosane C 100 H 202 hectane © E. V. Blackburn, 2005

n-Butane n- - specifies a straight chain hydrocarbon, e. g. nbutane or normal butane ? © E. V. Blackburn, 2005

Prefixes. . . iso- (CH 3)2 CH- isobutane © E. V. Blackburn, 2005

Pentane n-pentane isopentane neopentane © E. V. Blackburn, 2005

Hexane There are five alkane isomers of formula C 6 H 14. . . © E. V. Blackburn, 2005

n-hexane © E. V. Blackburn, 2005

isohexane © E. V. Blackburn, 2005

Neohexane © E. V. Blackburn, 2005

and. . . . © E. V. Blackburn, 2005

Nomenclature Why not name these more complex alkanes by first identifying and naming the longest carbon chain? – the parent chain. Then consider the groups attached to the parent chain as substituents? © E. V. Blackburn, 2005

Alkyl group substituents An alkyl group is the structure obtained when a hydrogen atom is removed from an alkane. These groups are named by replacing the -ane suffix of the corresponding alkane by -yl, hence “alkyl”. • CH 3 - methyl (Me-) • CH 3 CH 2 - ethyl (Et-) • (CH 3)2 CH- isopropyl (i-Pr) • CH 3 CH 2 - propyl (Pr) © E. V. Blackburn, 2005

Alkyl group substituants • CH 3 - methyl (Me-) • CH 3 CH 2 - ethyl (Et-) • (CH 3)2 CH- isopropyl (i-Pr) • CH 3 CH 2 - propyl (Pr) • CH 3 CH 2 CH 2 - butyl • (CH 3)2 CHCH 2 - isobutyl • but …. (CH 3)3 C- ? • or CH 3 CHCH 2 CH 3 | © E. V. Blackburn, 2005

alkyl group classification • a “primary” carbon is bonded to one other carbon • a “secondary” carbon is bonded to two carbon atoms • a “tertiary” carbon is bonded to three carbon atoms sec-butyl tert-butyl © E. V. Blackburn, 2005

IUPAC nomenclature • The longest continuous carbon chain forms the basic carbon skeleton. • If there are two of these chains, select the one with the greater number of branch points. • The remaining alkyl groups are considered as substituents. © E. V. Blackburn, 2005

Nomenclature • The carbon chain is then numbered from the end nearer the first branch point. • The different substituent groups are assigned numbers based on their positions along this chain. • Every substituent must have a number even if they are on the same carbon. • If identical substituents are present use the prefixes di-, tri-, tetra- etc. 2, 3 -dimethylpentane not 3, 4 -dimethylpentane © E. V. Blackburn, 2005

Substituents are named in alphabetical order. 4 -ethyl-3 -methylheptane © E. V. Blackburn, 2005

Hexane © E. V. Blackburn, 2005

Nomenclature of branched alkyl groups Numbering begins at the point where the group is attached to the main chain. © E. V. Blackburn, 2005

Nomenclature of branched alkyl groups © E. V. Blackburn, 2005

Nomenclature of alkyl halides © E. V. Blackburn, 2005

Nomenclature of alcohols Add the suffix ol to the name of longest, linear, carbon chain which includes the carbon bearing the OH and any double or triple C-C bond. The OH group has a higher priority than a multiple CC bond, a halogen, and an alkyl group in determining the carbon chain numbering. © E. V. Blackburn, 2005

Nomenclature of alcohols 3 -methyl-2 -butanol © E. V. Blackburn, 2005

Nomenclature of alcohols 2 -hydroxypropanoic acid © E. V. Blackburn, 2005

Other nomenclature systems 1. Name the alkyl group followed by the word alcohol: 2. Name alcohols as derivatives of carbinol, methanol: © E. V. Blackburn, 2005

Vicinal glycols “vicinal” means “adjacent” (vicinus, Latin for adjacent), “glycol” means “diol” Alcohols having two OH groups are called “glycols”: HOCH 2 OH is ethylene glycol or 1, 2 -ethanediol © E. V. Blackburn, 2005

Ethers Structure: R-O-R, Ar-O-R, or Ar-O-Ar nomenclature Name the two groups bonded to the oxygen and add the word ether. CH 3 CH 2 OCH 2 CH 3 - diethyl ether © E. V. Blackburn, 2005

Nomenclature of ethers diphenyl ether CH 3 OCH=CH 2 methyl vinyl ether isopropyl phenyl ether CH 3 CH 2 CHCH 2 CH 3 | OCH 3 3 -methoxyhexane © E. V. Blackburn, 2005

Nomenclature of cycloalkanes Cycloalkanes are named by adding the prefix cyclo to the name of the corresponding n-alkane. cyclopropane 1, 3 -dibromocyclohexane 1 -bromo-2 -chlorocyclopentane © E. V. Blackburn, 2005

Bicyclic compounds Use the name of the alkane corresponding to the total number of carbons in the rings as the parent: Seven carbons – a bicycloheptane. Now determine the number of carbons in each bridge and place them in the name in order of decreasing length. Bicylo[2. 2. 1]heptane! © E. V. Blackburn, 2005

Bicyclic compounds bicyclo[2. 1. 0]pentane bicylco[3. 1. 1]heptane Number the carbons beginning at one bridgehead, along the longest bridge, then the next longest back to the original bridgehead, then along the shortest bridge. 7 -methylbicyclo[2. 2. 1]heptane © E. V. Blackburn, 2005

Nomenclature of cyclic ethers Use the prefix oxa- to indicate that an O replaces a CH 2 in the ring. oxacyclopropane ethylene oxide oxacyclopentane tetrahydrofuran 1, 4 -dioxacyclohexane 1, 4 -dioxane © E. V. Blackburn, 2005

Nomenclature of alkenes 1. To name alkenes, select the longest carbon chain which includes the carbons of the double bond. Remove the -ane suffix from the name of the alkane which corresponds to this chain. Add the suffix -ene. a derivative of heptene not octane © E. V. Blackburn, 2005

Nomenclature of alkenes 2. Number this chain so that the first carbon of the double bond has the lowest number possible. © E. V. Blackburn, 2005

Nomenclature of alkenes © E. V. Blackburn, 2005

Butene - C 4 H 8 1 -butene The following are obviously butenes: 2 -butene methylpropene However there are four alkenes of formula C 4 H 8! compound A B C D bp mp -7 C -141 C -6 C < -195 C +1 C -106 +4 C -139 C © E. V. Blackburn, 2005

The butenes - C 4 H 8 compound A B C D bp mp -7 C -141 C -6 C < -195 C +1 C -106 +4 C -139 C “A” must be methylpropene! © E. V. Blackburn, 2005

The butenes - C 4 H 8 compound A B C D bp mp -7 C -141 C -6 C < -195 C +1 C -106 +4 C -139 C methylpropene “B” is 1 -butene © E. V. Blackburn, 2005

The butenes - C 4 H 8 compound A B C D bp mp -7 C -141 C -6 C < -195 C +1 C -106 +4 C -139 C methylpropene 1 -butene © E. V. Blackburn, 2005

2 -butene trans cis © E. V. Blackburn, 2005

Nomenclature Replace the -ane ending of the parent alkane with -yne. The numbering is analogous to that for alkenes. 1 -butyne 2 -butyne 4 -methyl-2 -pentyne © E. V. Blackburn, 2005

Nomenclature “Enynes” are compounds containing both a double and a triple bond. Numbering of the chain starts from the end nearer to the first multiple bond, be it double or triple. © E. V. Blackburn, 2005

Problems Try problems 4. 5, page 148, 4. 6, page 149, 4. 7 and 4. 8, page 151, and 4. 19 and 4. 20 on page 186 of Solomons and Fryhle. © E. V. Blackburn, 2005

Physical properties of alkanes and cycloalkanes • non-polar • low melting point (-183 C for methane) • low boiling point (-161. 5 C for methane) • colorless • insoluble in water • soluble in non-polar solvents such as petrol, ether, etc. © E. V. Blackburn, 2005

Cyclopropane © E. V. Blackburn, 2005

Cyclobutane © E. V. Blackburn, 2005

Relative stabilities of cycloalkanes Baeyer (1885) proposed that rings smaller and larger than cyclopentane were unstable due to angle strain. How does this hypothesis fit the facts? Angle strain in cyclic compounds can be quantitatively evaluated by comparing heats of combustion for each -CH 2 - unit. © E. V. Blackburn, 2005

Heats of combustion/CH 2 Cyclane (CH 2)n n n-alkane H/n (k. J) 658. 6 cyclopropane 3 697. 0 cyclobutane 4 686. 0 cyclopentane 5 664. 0 cyclohexane 6 658. 7 cycloheptane 7 662. 4 cyclooctane 8 663. 8 cyclopentadecane 15 free of angle strain 659. 0 free of angle strain!!! Why? © E. V. Blackburn, 2005

Cyclanes have puckered, not flat rings: cyclobutane cyclopentane cyclohexane © E. V. Blackburn, 2005

Conformational analysis angle strain Any atom tends to have bond angles that match those of its bonding orbitals: 109. 5 o for sp 3 -hybridized carbons. Any deviation from these normal bond angles is accompanied by angle strain. © E. V. Blackburn, 2005

Conformational analysis torsional strain Any pair of sp 3 carbons bonded to each other tend to have their bonds staggered. Any deviation from the staggered conformation is accompanied by torsional strain. © E. V. Blackburn, 2005

Conformational analysis - van der Waals strain Non-bonded atoms that just touch one another attract each other. If they are closer, they repel each other. Such crowding is accompanied by van der Waals strain (steric strain). © E. V. Blackburn, 2005

Cyclohexane - the “chair” conformation © E. V. Blackburn, 2005

The “boat” conformation 1. 83 A This conformation is less stable (29. 7 k. J/mol) than the chair conformation. It is situated at the top of a PE curve and is therefore a transition state between 2 conformational isomers. © E. V. Blackburn, 2005

Skew-boat conformations The skew-boat conformations are 23. 0 k. J/mol less stable than the chair conformation. © E. V. Blackburn, 2005

Conformations of cyclohexane © E. V. Blackburn, 2005

Axial and equatorial hydrogens © E. V. Blackburn, 2005

Axial and equatorial hydrogens axial equatorial © E. V. Blackburn, 2005

Methylcyclohexane equatorial © E. V. Blackburn, 2005

Methylcyclohexane - axial © E. V. Blackburn, 2005

trans-1, 2 -dimethylcyclohexane © E. V. Blackburn, 2005

cis-1, 2 -dimethylcyclohexane © E. V. Blackburn, 2005

cis v trans © E. V. Blackburn, 2005

cis-1, 3 - © E. V. Blackburn, 2005

trans-1, 3 - © E. V. Blackburn, 2005

trans-1, 4 - © E. V. Blackburn, 2005

cis-1, 4 - © E. V. Blackburn, 2005

Nomenclature © E. V. Blackburn, 2005

Problems Try problems 4. 13 (page 174), 4. 14 (page 175), and 4. 41 - 4. 42 (page 190) in the textbook. © E. V. Blackburn, 2005

Synthesis of alkanes and cycloalkanes © E. V. Blackburn, 2005

Hydrogenation of alkenes and alkynes © E. V. Blackburn, 2005

Reduction of alkyl halides © E. V. Blackburn, 2005

Alkylation of terminal alkynes An acetylenic hydrogen is weakly acidic: © E. V. Blackburn, 2005

Alkylation of terminal alkynes The anion formed will react with a primary halide: © E. V. Blackburn, 2005

Corey – Posner – Whitesides House Synthesis © E. V. Blackburn, 2005

Retrosynthetic analysis Here is a target molecule. Plan a synthesis. © E. V. Blackburn, 2005

Retrosynthetic analysis © E. V. Blackburn, 2005

Corey – Posner – Whitesides House Synthesis Muscalure is the sex pheromone of the common house fly. It is used to attract flies to traps containing insecticide. It can be synthesized by the Corey - House reaction. What lithium dialkylcuprate would you use? Muscalure © E. V. Blackburn, 2005

Problems Try problem 4. 45, page 190 of Solomons and Fryhle. © E. V. Blackburn, 2005

Reactions of alkanes with halogens © E. V. Blackburn, 2005

Chlorination - a substitution reaction © E. V. Blackburn, 2005

Polychlorination CH 3 Cl + Cl 2 CH 2 Cl 2 + HCl dichloromethane methylene chloride CHCl 3 + HCl trichloromethane chloroform CHCl 3 + Cl 2 CCl 4 + HCl tetrachloromethane carbon tetrachloride © E. V. Blackburn, 2005

A Problem? Chlorination leads to the possible formation of four products - a mixture! How can we limit the reaction so that only one product is formed? © E. V. Blackburn, 2005

Bromination takes place less readily than chlorination but it produces the four analogous brominated products: • bromomethane • dibromomethane -methylene bromide • tribromomethane - bromoform • tetrabromomethane - carbon tetrabromide © E. V. Blackburn, 2005

Iodination and fluorination • iodine does not react • fluorine reacts very readily order of halogen reactivity: © E. V. Blackburn, 2005

A Mechanism • a detailed, step by step, description of the transformation of reagents into products • it must explain all experimental facts • the mechanism should be tested by devising appropriate experiments - mechanistic predictions must be tested in the lab © E. V. Blackburn, 2005

Mechanism of the chlorination of methane The experimental facts 1. No reaction occurs at room temperature in the absence of light. 2. Reaction readily occurs, in the absence of light, at temperatures above 250 C. 3. Reaction occurs at room temperature in the presence of light of a wavelength absorbed by chlorine. © E. V. Blackburn, 2005

The experimental facts 4. When the reaction is initiated by light, a large number of chloromethane molecules are produced for each photon of light absorbed by the system. 5. The presence of even a small quantity of oxygen slows down the reaction. © E. V. Blackburn, 2005

Inhibitors A compound which slows down or stops a reaction, even when present in small quantities, is called an inhibitor. © E. V. Blackburn, 2005

Lets test the mecanism If tetraethyllead is heated at 140 C. . . F. Paneth and W. Hofeditz, Ber. , 62, 1335 (1929) © E. V. Blackburn, 2005

Chlorination H = - 102 k. J. . H 243 k. J 438 k. J 243 k. J 432 k. J + 6 k. J 351 k. J - 108 k. J © E. V. Blackburn, 2005

How does Cl. react with CH 4? In order for chlorination to occur, a Cl. and a CH 4 must collide. The H-Cl bond can only form if the two species come in contact. A certain minimum energy must be provided by the collision in order for reaction to occur. Why? ? ? © E. V. Blackburn, 2005

Activation energy Bond breaking and bond formation are not perfectly synchronous processes. Therefore energy liberated during bond formation is not completely available for bond breaking. A collision must therefore provide a certain minimum amount of energy for reaction to occur. This is called the “activation energy”, Ea. © E. V. Blackburn, 2005

Factors affecting collision frequency • concentration • pressure • molecular size • momentum • temperature © E. V. Blackburn, 2005

The probability factor • depends on reactant geometry • depends on the nature of the reaction taking place © E. V. Blackburn, 2005

Relative rates of reaction At 275 C, of every 15 million collisions, 375, 000 are of sufficient energy to cause reaction when chlorine atoms are involved … and only one is of sufficient energy when bromine atoms are involved. Thus, solely due to Ea differences, the chlorine atom is 375, 000 more reactive than the bromine atom. © E. V. Blackburn, 2005

The intermediate alkyl radical The nature of the intermediate free radical determines the product: © E. V. Blackburn, 2005

Orientation of halogenation abstraction of a primary hydrogen abstraction of a secondary hydrogen We have competing reactions and should review factors which influence reaction rates! © E. V. Blackburn, 2005

Probability factor The statistical product ratio for the chlorination of propane is 75% 1 -chloropropane and 25% 2 -chloropropane, a 3: 1 mixture. Why? There are three times as many primary hydrogens. However: © E. V. Blackburn, 2005

Relative reactivities Lets look at the relative reactivities per hydrogen atom: tertiary secondary primary Chlorination: 5. 0 : 3. 8 : 1. 0 Bromination: 1600 : 82 : 1 We need to look at activation energies and transition states! © E. V. Blackburn, 2005

Free radical stability H 3 C-H CH 3. + H. H = 438 k. J CH 3 CH 2 -H CH 3 CH 2. + H. H = 420 k. J (CH 3)2 CH-H (CH 3)2 CH. + H. H = 401 k. J (CH 3)3 C-H (CH 3)3 C. + H. H = 390 k. J Order of free radical stability is therefore tertiary > secondary > primary > methyl © E. V. Blackburn, 2005

Free radical stability hyperconjugation Using the concept of resonance: - A charged system is stabilized when the charge is dispersed or delocalized. Thus the order of free radical stability is tertiary > secondary > primary > methyl. © E. V. Blackburn, 2005

Free radical stability hyperconjugation The electrons are delocalised through overlap of a p orbital which is occupied by one, lone electron, and a orbital of the alkyl group: ethyl radical © E. V. Blackburn, 2005

Transition state for rate determining step Factors which stabilize free radicals will stabilize the transition state which is developing free radical character. © E. V. Blackburn, 2005

Orientation of halogenation This is determined by the stability of the transition state for the rate determining step. © E. V. Blackburn, 2005

Reactivity and selectivity and the Hammond postulate The postulate states that the transition state resembles the structure of the nearest stable species. Transition states for endothermic steps structurally resemble products whereas transition states for exothermic steps structurally resemble reactants. Thus the later the transition state is attained in the reaction, the more it resembles the products. In other words, the greater the Ea, the more the transition state resembles the products. This will explain the greater selectivity of the bromine atom. © E. V. Blackburn, 2005

Reactivity and selectivity This reaction has a low activation energy and so the transition state resembles the reactants - it has little radical character. The activation energy for the bromination is far higher. The transition state has considerable radical character. The free radical stabilizing factors are far more important in the bromination, hence the greater selectivity. © E. V. Blackburn, 2005