ERT 102 ORGANIC CHEMISTRY

ALKENES

Dr. Syazni Zainul Kamal

School of Bioprocess Engineering

Alkenes

•Hydrocarbons that contain a carbon-carbon doublebond.

•Important roles in biology, e.g ethene (H2C=H2C)plant hormone- affect seed germination, flowergermination/fruit ripening.

Shipped green. Ripening start when exposed to ethene

Molecular formula of alkene

•CnH2n (2 fewer H than alkane)

•The general formula for cyclic alkene: CnH2n-2

minus 2 H for every p bond

•Double bond is the functional group of analkene, its presence is denoted by the suffix“ene”. Eg ethene, propene.

Nomenclature of alkenes

1. Number the longest continuous chaincontaining the functional group in the directionthat gives the functional group suffix the lowestpossible number

2. For compound with 2/more double bonds, the suffix is ‘diene’, ‘triene’, ‘tetraene’

1,3,5-hexatriene

3. Name of substituent is stated before the nameof the longest continuous chain that contains thefunctional group, together with a no. to designatethe carbon to which the substituent is attached.

4. If a chain has more than 1 substituent, thesubstituents are stated in alphabetical order.Appropriate no is assigned to each substituent.

5. If counting in either direction results in thesame no. for the alkene functional group suffix,the correct name is the one containing the lowestsubstituent number

6. A number is not needed to denote the position of thedouble bond in a cyclic alkene - the ring is alwaysnumbered so that the double bond is between C1 andC2.

- To assign number to substituents, count around the ringin the direction that put the lowest no into the name.

3-ethylcyclopentene 4,5-dimethylcyclohexene 4-ethyl-3-methylcyclohexene

7. If counting in both direction give same no. for functional group and the same lowest no. for one or more substituent, choose direction that give lowest no to remaining substituent

Try this!

Exercises

•What is the IUPAC name for the following compound?

A) 5-methylcyclohexene

B) 4-methylcyclohexene

C) 1-methyl-3-cyclohexene

D) 1-methyl-4-cyclohexene

E) methylcyclohexene

•Name of the following compound:

2-ethyl-1-pentene

2-methyl-1,4-pentadiene

2-methyl-1-butene

Isomer of alkene

Alkenes Can Have Cis And Trans Isomers

•Rotation about a double bond occurs only if the πbond breaks

•Cis and Trans isomers have same mol. Formula &same bond but diff. configuration

•H on the same side of double bond- cis isomer•H on the opposite side of double bond- trans isomer

• If one of the sp2 C is attached to 2 identicalsubstituents, there is only 1 possible structure for thealkenes.

•Because of the energy barrier- cis and trans isomers of alkenes cannot interconvert (except under extreme conditions that can overcome the energy barrier)

•They can be separated from each other

•The 2 isomers are different compounds with different physical properties ie. bp

using the E,Z system to distinguish isomers

•as long as each of the sp2 carbon of an alkene isbonded to only 1 hydrogen, we can use the termcis and trans to designate the structure of alkenes

How to designate the isomers such as 1-bromo-2-chloropropene?

E,Z system of nomenclature

•To name an isomer by E, Z system:

- determine the relative priorities of the 2 groupsbonded to 1 sp2 C

- Then, the relative priorities of the 2 groupsbonded to the other sp2 carbon.

E,Z is from German:

Z, Zusammen (together) E, Entgegen (opposite)

Determining relative priorities

1. Relative priorities of the 2 groups depend on the atomicnumber of the atoms bonded directly to the sp2 C. thegreater the no, the higher priority.

2. If two atoms attached to sp2 C are the same (there is a tie),consider the atomic no. of the atoms attached to tiedatoms.

Left Sp2 C – bonded to C C of CH2Cl group bonded to Cl, H, H C of the CH2CH2Cl group is bonded to C, H, H

Right Sp2 C – bonded to C C of CH2OH group bonded to O, H, H C of the CH(CH3)2 group is bonded to C, C, H

3. If an atom is doubly bonded to another atom, treats it asif it were singly bonded to two of those atoms.

If an atom is triply bonded to another atom, treats it as if itwere singly bonded to 3 of those atoms

- Atom bonded to sp2 c are both C, H, H. There are tie. ignorethem, look at the groups that attached to CH2

C of CH2OH group bonded to O, H, H C of the C CH group is bonded to C, C,C

C of CH=CH group bonded to C, C, H C of the CH2CH3 group is bonded to C, H, H

4. If 2 isotopes (atoms with same atomic number butdifferent mass no) are compared, mass number is used todetermine priorities.

- Deuterium (D) and H have same atomic number butdifferent mass, D has higher priority than H.

- the C that are bonded to the other sp2 C are bothbonded to C, C and H so the next atoms are looked to.

Draw the structure of (E)-1-bromo-2-methyl-2-butene

Draw the compound without specify the isomer. Can know what substitute bonded to sp2 carbons

Determine relative priorities of 2 groups bonded to sp2 C

Left sp2 C bonded to CH3 and CH2Br

Right sp2 C bonded to H and CH3

How alkenes react

Electron-rich and electron-deficient species

•Electrophiles – electron deficient species (e- loving), has either positive charge, partial positive charge or incomplete octet

•Nucleophile – electron rich species , has electron pair to share

Mechanism of Alkenes reaction

•Alkenes are more reactive than alkanes due to thepresence of double bond.

•Alkene is a nucleophile and susceptible to beattacked by electrophiles (electron deficientspecies).

•Alkenes undergo ADDITION reaction which meansthe C=C are broken to form C-C bonds.

Addition Of A Hydrogen Halide To An Alkene

•hydrogen halide (electrophile) the product of thereaction will be an alkyl halide:

•Alkenes in these reactions have the same substituents onboth sp2 carbons

•The electrophile (H+) adds to 1 of the sp2 carbons, andthe nucleophile (X-) adds to the other sp2 carbon- doesn’tmatter to which C it will attach to- same product.

What Happen- If Alkene Does Not Have The Same Substituents on both sp2 C?

Since the only product formed is tert-butyl chloride-tert-butyl cation is formed faster than isobutyl cation.

carbocation

1st step - Addition of H+ to sp2 C to form either one of the productSlow-rate determining stepCarbocation that is form faster will predominate

Why is the tert-butyl cation formed faster?

•Carbocations are classified according to the carbon thatcarries the +ve charge.

•Carbocation's stability depends on the number of alkylgroups attached to the +vely charged carbon.

-Primary carbocation = +ve charge on primary C-Secondary carbocation = +ve charge on secondary C-Tertiary carbocation = +ve charge on tertiary C

•Thus, the stability of carbocations increases as the no of alkyl substituent attached to +vely charged carbon increases.

Alkyl groups decrease the concentration of positive charge- makes the carbocation more stable!

Tertiary carbocation – more stable

Primary carbocation

In an electrophilic addition reaction, the more stable carbocation is formed more rapidly

Major product is the one obtained by adding electrophile to the sp2 C that result in formation of more stable carbocation

Exercise

•Which of the following is the most stable carbocation?

•Rank from most stable to least stable

Electrophilic addition reactions are regioselective

•Major product of electrophilic addition reaction is the one obtained by adding electrophile to the sp2

carbon that result in formation of more stable carbocation sp2 C that do not get proton is the one that is

+ve charge in the carbocation intermediate

• Both reaction form 2products• Major product – reaction of nucleophile with more stablecarbocation (carbocation that form rapidly)•The products known as constitutional isomers – samemolecular formula, differ in how their atoms are connected.•regioselective reaction – reaction that form constitutionalisomer, but one of them predominates

Degrees of regioselectivity

•Regioselectivity is the preferential formation ofone constitutional isomer over another

•3 degrees of regioselective:

Moderately regioselective,

highly regioselective,

completely regioselective

- E.g: addition of a HCl to 2-methylpropene (2 possiblecarbocations are tertiary and primary) is moreregioselectivity than

- Addition of a HCl to 2-methyl-2-butene (2 possiblecarbocations are tertiary and secondary) - closer instability.

•Following reaction is not regioselective

•Addition of H+ to either sp2 C produce secondary carbocation

•Equal amount of two alkyl halides are obtained

•Markovnikov’s rule: the electrophile adds preferentially tothe sp2 C bonded to the most hydrogens•Quick way to determine to which sp2 C the electrophile

add 1st

• In the above reaction, the electrophile (H+) addspreferentially to C-1 because C-1 bonded to 2 H.•Or we can say that: H+ adds to C-1 because it results in the

formation of secondary carbocation, which is more stablethan primary carbocation- would be formed if H+ added toC-2.

Rule that determines the product of an electrophilic addition reaction

•What is the major product obtained from the addition of HBr to following compound?

•What alkene should be used to synthesize 3-bromohexane?

Solution:

1. List the potential alkenes that can be used to produce 3-bromohexane. Since you want to produce alkyl halide that has bromo substituent at 3-position, alkene should have sp2 C at that position

2. Since there are 2 possibilities- decide whether there is any advantage of using 1 over the other

- The addition of H+ to 2-hexene: form 2 differentsecondary carbocations, same stability- equalamounts of each will be formed. ½ 3-bromohexaneand ½ 2-bromohexane.

- The addition of H+ to either of the sp2 carbons of 3-hexene- forms the same carbocation because thealkene is symmetrical. Thus, all product will be 3-bromohexane.

- Therefore, 3-hexene is the best alkene to use toprepare 3-bromohexane.

A Carbocation Will Rearrange If It Can Form A More Stable Carbocation

•Sometimes, in the electrophilic addition reactions, theproducts obtained are not as expected.

•For eg: the addition of HBr to 3-methylbutene forms 2products.

- 2-bromo-3-methylbutane - predicted

- 2-bromo-2-methylbutane - unexpected product

expected unexpected

•F.C. Whitmore- 1st to suggest that the unexpectedproducts results from a rearrangement of thecarbocation intermediate.

•Not all carbocation rearrange

•Carbocations rearrange if they become more stable asa result of the rearrangement.

•1,2-hydride shift

•1,2-methyl shift

• Initially – secondary carbocation formed - has H that can shift its pair of electron to the neighbour +ve charge C -creating stable tertiary carbocation

•Result of the carbocation rearrangement- 2 alkyl halides areformed

1,2-hydride shift (H move from 1 C to neighbor C)

•Rearrangement is so fast

• Little unarranged carbocation exists in solution

•Major product is the result from adding nucleophile to the rearranged carbocation

•Initially secondary carbocation formed.•Methyl group with its pair electron shift to neighbour +vecharge C to form stable tertiary carbocation•2 product form – major (addition of nucleophile torearranged carbocation), minor (add of nucleophile tounarranged carbocation)

1,2-methyl shift1,2-methyl shift (methyl move from 1 C to neighbor C)

• If a rearrangement does not lead to a more stablecarbocation, then the rearrangement does not occur.

•For eg: when a proton adds to 4-methyl-1-pentene, asecondary carbocation is formed.

•A 1,2-hydride shift would form a different secondarycarbocation- but since both carbocations are equallystable-no advantage to the shift. Rearrangement doesnot occur.

•Which of the following carbocations would be expected to rearrange?

A is secondary carbocation

B is secondary carbocation. 1,2-hydride shift converts it to tertiary carbocation

C is tertiary carbocation

D is tertiary carbocation

E is secondary carbocation. 1,2-hydride shift converts it to tertiary carbocation

F is secondary carbocation

The Addition Of A Halogen To An Alkene

•The halogens Br2 and Cl2 add to alkenes.

•Product = vicinal dibromide/dichloride

•Reaction carried out by mixing with inert solvent eg. dichloromethane (not participate in reaction)

Special reactionBond of halogen is weakAlkene induce dipole

Mechanism For The Addition Of Bromine To An Alkene

• p electrons of the alkene approach Br2

•one of the Br atoms accepts those electrons and releasesshared electrons to the other Br atom (bromine ion)•Because Bromine electron cloud close to other sp2 C, acyclic bromonium ion intermediate is formed•3-membered ring - unstable•Thus, the cyclic brominium ion reacts with a nucleophile(Br-)• product is vicinal dibromide

• The product for 1st step: cyclic bromonium ion. NOTcarbocation- Br electron cloud is close to the other sp2

carbon- form a bond.• Bromonium ion more stable- its atom have completeoctets.• Positively charged carbon of carbocation – does nothave complete octet.

Halohydrin Formation

• If H2O used as solvent- major product will be a vicinal halohydrin (bromohydrin/chlorohydrin)

•Halohydrin- organic molecule that contains both halogen and an OH group (bonded to adjacent C)

Mechanism for halohydrin formation

• 1st step: a cyclic bromonium ion is formed because Br+/Cl+

the only electrophile in the reaction mixture• 2nd step: the unstable cyclic bromonium ion rapidly reacts

with any nucleophile it bumps into. Two nucleophilespresent: H2O and Br-, but because H2O is the solvent, itsconc > Br-. Tendency to collide with H2O is more.

• The protonated halohydrin is strong acid- so it losesproton.

•When nucleophile other than water used:

i) Electrophile add to the sp2 C that is bonded to the most H

ii) Nucleophile add to the other sp2 C

Addition of water to alkene

•Alkene can’t react with water (no electrophile to add to alkene)

•Add acid (H2SO4), provide electrophile

•Product of reaction is alcohol

•Hydration reaction (addition of water to a mol.)

•H+ (electrophile) add to sp2 C of an alkene (nucleophile) that is bonded to most H2

•H2O (nucleophile) add to carbocation (electrophile), form protonated alcohol

•Protonated alcohol lose a proton to H2O and form alcohol and hydronium ion

Mechanism for acid-catalyzed addition of water to alkene

Addition of alcohol to alkene

•Reaction with alcohol also require acid catalyst

•Product of reaction is ether

• H+ (electrophile) add to sp2 C of an alkene (nucleophile) that is bonded to most H2

• CH3OH (nucleophile) add to carbocation (electrophile), form protonated ether

• Protonated ether lose a proton

Mechanism of acid-catalyzed reaction of alcohol to alkene