Elimination Reactions - Organic Chemistry - Lecture Notes, Study notes of Organic Chemistry

Download Elimination Reactions - Organic Chemistry - Lecture Notes and more Study notes Organic Chemistry in PDF only on Docsity! 1 Alkenes; Elimination Reactions Alkenes Alkenes contain a carbon-carbon double bond. They are named as derivatives of alkanes with the suffix “-ane” changed to “-ene”. The parent alkane is the longest continuous chain that contains the double bond. The double bond is specified by the number of the first carbon in the double bond and chain containing the double bond is numbered so as to give the first carbon the lowest number. CH2 CH CH2CH3 1 2 3 4 1-butene CH CH CH3 1 2 3 4 CH3 2-butene CH2 CH2 ethene (or ethylene) CH2 CH CH3 propene (or propylene) The double bond takes precedence over alkyl and halogen substituents. CH2 CH CHCH3 2 3 4 CH3 1 3-methyl-1-butene Br CH2 CH2 CH2 CH CH CH2CH2CH3 CH2 12 3456 6-bromo-3-propyl-1-hexene Carbon-carbon double bonds take precedence over alkyl groups and halogens but hydroxyl groups take precedence over the double bond and so get the lowest number. The number specifying the position of the double bond is put before the number specifying the position of the hydroxyl group. C C CH2CH3 CH3 Cl HOCH2CH2CH2 1 2 3 4 5 6 7 4-chloro-5-methyl-4-hepten-1-ol. or: 4-chloro-5-methylhept-4-en-1-ol. Some common names: CH2 CH CH2 CH CH2 vinyl group allyl group CH2 C CH3 isoprenyl group CH2 CH CH2 OH allyl alcohol CH2 C Cl CH3 When a –CH2 is doubly bonded to a ring, use methylene. docsity.com 2 CH2 methylenecyclohexane Cycloalkenes: There is no need to specify the position of the double bond since it will automatically get number one. Then number around the ring so as to give the lowest number to the substituents at the first point of difference. cyclopentene CH3 1-methylcyclohexene Cl CH2CH3 1 2 3 4 5 3-chloro-5-methylcycloheptene Review of Structure and Bonding C C H H H C H H H sp2 carbon 1.34 A° 1.50 A° C C HH HCH3 The p orbitals of the π−bond are perpendicular to the plane of the sigma framework. Cis and Trans Isomerism There is no free rotation around a carbon-carbon double bond and so alkenes can exist as cis or trans isomers. C C H CH3 H CH3 cis-2-butene C C H CH3 CH3 H trans-2-butene The cis or trans nomenclature is useful when the two substituents are the same or similar but what about the following? C C Br F Cl H For molecules such as this we need a new system, E or Z, that is based on the atomic number. Rules for assigning the alkene configuration as E or Z: docsity.com 5 Physical Properties Alkenes are generally non-polar, like alkanes, and have fairly low boiling points. Alkenes are electron deficient. They are missing two electrons as compared to alkanes. H C H H C H H H 14 electrons C C HH H H 12 electrons Alkyl groups donate electrons to the electron deficient alkene to help stabilize it. The alkyl group is a much better electron donor than a hydrogen. C C HH3C H H From measuring heats of combustion, we learn that alkyl substituents on a double bond help to stabilize it. The more substituted with electron donating alkyl groups, the more stable it is. C C RR R R > C C RR H H > C C HR H H > C C HR H R > C C HR R H > C C RR R H > C C HH H H tetra-substituted trisubst. 1,1-disubst. trans-1,2-disubst. cis-1,2-disubst. mono-subst. unsubst. most stable least stable This donation of electrons by the alkyl to stabilize the double bond is called an electronic effect. There is also a steric effect due to van der Waals strain. The cis-isomer is slightly higher in energy than the trans, since the two alkyl groups are large and the electrons repel each other slightly. Note that the alkyl groups lie in the same plane and so are eclipsed in the cis-isomer. As the alkyl groups become larger, the steric hindrance becomes larger and the difference in energy between the cis- and trans-isomers also becomes larger. C C CH3CH3 H H C C HCH3 H CH3 3 KJ/mol lower in energy C C CC H H C C HC H C 44 KJ/mol lower in energy CH3 CH3 CH3 CH3 CH3 CH3 CH3CH3 CH3 CH3 CH3 CH3 Cycloalkenes docsity.com 6 Cyclopropene exists and is actually found in nature. It is highly strained. The smallest ring size that will contain a trans-double bond is eight. cyclopropene cis-cyclooctene trans-cyclooctene 39 KJ/mol higher in energy than the cis-isomer. Not stable above -90°C. stable but highly strained Elimination Reactions: Preparation of Alkenes These are called β−eliminations or 1,2 eliminations. C X C Y βα C C + X Y X and Y must be on adjacent carbons. Dehydration of Alcohols When alcohols are heated with a catalytic amount of acid, they lose water to form alkenes. The acid is not used up but is catalytic in that it speeds up the reaction and is reovered to be used again. R4 C OH R3 C R1 H R2 βα C C R1 R2 R4 R3 H+ + H O H + H O H H heat OH H H2SO4 heat + H2O We see that tertiary alcohols react faster than secondary, which react faster than primary alcohols. (3° > 2° > 1°) With unsymmetrical alcohols, we see that the more substituted alkene is usually formed as the major product. This is called Zaitsev’s rule. The β−hydrogen is removed from the carbon with the least number of hydrogens so as to form the alkene that has more alkyl substituents. The alkene which has the more electron donating alkyl groups is the more stable alkene. docsity.com 7 CH3 C OH CH3 CH2CH3 H2SO4 heat CH2 C CH2CH3 CH3 + CH C CH3 CH3 CH3 10% 90% disubstituted double bond trisubstituted double bond; more stable When there are two possible products of a reaction and one is formed in greater amounts, we say that this reaction is regioselective. When two stereoisomers are possible and one of them is formed in greater amounts, we call this reaction stereoselective as in the following example. We see that the trans-isomer is the major product because it is more stable. This is a general trend: usually in acidic condition, when the reaction is reversible, the major product will be the more stable product. CH3CH2 C OH H CH2CH3 H2SO4 heat C C CH2CH2 HH CH3 cis-2-pentene (25%) + C C CH2CH2 HCH3 H trans-2-pentene (75%) Mechanism There are two mechanisms that occur, depending on the substrate. With tertiary substrates we see the E1 mechanism that involves a carbocation and is analogous to the SN1 mechanism with tertiary alcohols. With primary substrates we see the E2 mechanism that does not involve a carbocation and is analogous to the SN2 reaction. With secondary alcohols we usually see the E1 mechanism with the carbocation intermediate. E1 Mechanism The E1 mechanism is observed with tertiary alcohols and is similar to the SN1reaction of tertiary alcohols with hydrogen halides. Both reactions are catalyzed by acid and both involve a carbocation intermediate. The E1 reaction occurs in three steps, like the SN1 reaction. The first two steps of the E1 and SN1 reactions of tertiary alcohols are identical. It is the third step that is different. docsity.com 10 CH3 C CH3 CH3 CH CH3 OH H3PO4 CH3 C CH3 CH3 CH CH3 O H H CH3 C CH3 CH3 CH CH2 secondary carbocation CH2 C CH3 C CH3 CH3 tertiary carbocation H OH2 CH3 C CH3 CH3 CH CH2 fastslow very minor product H H OH2 C C CH3 CH3 CH3 CH3 major H2O C CH CH2 CH3 CH3 CH3 minor In the transition state for the methyl migration, the methyl group is partially bonded to both carbon atoms. CH3 C CH3 CH3 CH CH3 Methyl group migration CH3 C CH3 C H CH3 H3 C δ+δ+ secondary carbocation CH3 C CH3 C H CH3 CH3 tertiary carbocation These rearrangements reactions are very fast. There is a small energy barrier to the migration. The empty p orbital of the carbocation overlaps with the adjacent C-H sigma bond. CH3 C CH3 CH2 CH3 CH3 overlap of the sigma C-H bond and the empty p orbital of the carbocation We can also have migration of a hydride with its two electrons. CH3 C CH3 H C OH H CH3 H2SO4 CH3 C CH3 H C O H CH3 H H CH3 C CH3 H C H CH3 secondary carbocation CH2 C CH3 C H CH3 H tertiary carbocation H H2O OH2 C C H CH3 CH3 CH3 major C C CH2CH3 CH3 H H minor docsity.com 11 With primary alcohols, we do not see a carbocation but there can still be rearrangements. The migration of the alkyl group or hydride occurs at the same time as loss of the protonated alcohol. CH3CH2CH2 C CH3 CH3 C OH H H H2SO4 CH3CH2CH2 C CH3 CH3 C O H H H H CH3CH2CH C CH3 C CH3 H H H H2O OH2 C C CH3 H CH3CH2CH2 CH3 C C CH2CH3 CH3 CH3CH2 H methyl migration tertiary carbocation CH3CH2CH2 C H H C OH H H H2SO4 CH3CH2CH2 C H H C O H H H H CH3CH2CH C H C H H H H H2O OH2 C C H H CH3CH2CH2 H C C H CH3 CH3CH2 H Hydride migration major minor secondary carbocation Dehydrohalogenation Dehydrohalogenation occurs in basic conditions with loss of H-X to make an alkene. R1 C H R2 C R3 R4 X αβ B C C R3 R4 R1 R2 + X + BH A relatively strong base such as sodium ethoxide (NaOCH2CH3) in ethanol as the solvent is needed to remove the β-hydrogen. docsity.com 12 CH3 C H H C H Cl CH3 Na+ -:OCH2CH3 HOCH2CH3 C C CH3 CH3 H H We usually form the more substituted alkene according to Zaitsev’s rule. CH3 C H CH3 C H Cl CH2 K+ -:OCH2CH3 HOCH2CH3 70°C 29% C C H CH3 CH3 CH3 71% H A Product A Product B C C H H CH H CH3 CH3+ B And the trans or E-alkene is also usually favored since it is lower in energy due to the fact that it is less hindered. CH3CH2 C H H C H Br C H H CH2CH3 K+ -:OCH2CH3 HOCH2CH3 70°C C C CH2CH3CH3CH2CH2 HH 20% + C C CH2CH3H HCH3CH2CH2 80% Mechanism for Dehydrohalogenation This is an E2 reaction. Some experimental facts: (1) The reaction follows second order kinetics, meaning there are two reactive species in the transition state that participate in the reaction. Rate = k [alkyl halide] [base] (2) The rate increases with decreasing strength of the R-X bond. R I > R Br > R Cl > R F strongest C-X bond; slowest elimination reactionweakest C-X bond;fastest elimination reaction Rate Four things happen all at once (1) Formation of a new bond between the base, B, and the β-hydrogen. (2) Breaking of the β C-H bond. (3) Formation of the new carbon-carbon double bond. docsity.com 15 H H H C CH3 CH3 CH3 H Br trans-1-bromo-4--butylcyclohexane Br C CH3CH3 CH3 H H H CH3O OCH3 C CH3CH3 CH3 +C CH3CH3 CH3 same Isotope Effects Using a deuterium instead of hydrogen can give information on the mechanism of a reaction. Deuterium undergoes the same reactions as hydrogen but the rates of the reaction may be different. A carbon-deuterium is ~12 KJ/mol stronger than a carbon-hydrogen bond. Therefore, there is a slightly greater activation energy for breaking a C-D bond than for a C-H bond. Therefore the rate constant for a step in which a C-D bond is broken is smaller for for a C- H bond. This is called a kinetic isotope effect where the rate of the reaction using hydrogen (KH) is greater than the rate when using a deuterium (KD) by a factor of 3-8 (KH/KD= 3-8). For an E2 reaction, in which a C-H bond (or a C-D) bond is broken in the rate determining step, there is a kinetic isotope effect of 6.7. CH3 CH Br CH3 versus CD3 CH Br CD3 KH/KD = 6.7 E1 Mechanism Just the substitution reaction can occur in one step (SN2) or two steps (SN1), the elimination reaction also can occur by means of two different mechanisms. As discussed above, the E2 mechanism is bimolecular and occurs in one step. The E1 reaction is analogous to the SN1 mechanism and occurs in two steps by means of a carbocation intermediate. docsity.com 16 We see the E1 mechanism when we have a tertiary substrate and a weak base. Very often the base is the solvent (an alcohol or water) and we call this a solvolysis reaction. Very often we see a combination of E1 and SN1 reactions occurring together. For example: CH3 C CH3 Br CH2CH3 CH3CH2OH heat CH3 C CH3 OCH2CH3 CH2CH3 CH3 C CH2 CH2CH3 CH3 C CH3 CHCH3+ + The mechanism: The first step is the formation of a carbocation. This is a unimolecular step. The bromine leaves to form a carbocation that is stabilized by the solvent. Now the solvent ethanol can attack the carbocation directly in an SN1 reaction or it can act as a base to remove an α−proton to form an alkene. Two α−protons are available. The major alkene product will be the more substituted alkene in accordance with Zaitsev’s rule, since it is the more stable product. The proportion of substitution will depend on the substrate, the base and the reaction conditions but usually there is a mixture of both products. Higher temperature favors elimination, since it has the higher activation energy. Note also that rearrangements can occur in E1 conditions, since there is a carbocation intermediate. CH3 C CH3 Br CH2CH3 CH3 C CH2 CHCH3+Br H H HOCH2CH3 CH3 C CH3 CHCH3 CH3CH2OH CH3 C CH2 CH2CH3 minor CH3CH2OH CH3 C CH3 CH2CH3 O CH2CH3HCH3CH2OH CH3 C CH3 OCH2CH3 CH2CH3 E1 reactions are not too useful synthetically. If the elimination product is the desired goal, then a strong base is generally used so as to favor the E2 mechanism. docsity.com