PTT 102ORGANIC CHEMISTRY

SEM I 2012/2013

ALKENE

Rozaini Abdullah

School of Bioprocess Engineering

UniMAP

Week 3

Structure, nomenclature of alkenes and naming alkenes using the E,Z system:

- DEFINE and ILLUSTRATE the principle in naming alkenes on few examples

Alkenes Hydrocarbons that contain a carbon-carbon double

bond. Important roles in biology, e.g ethene- plant hormone-

effect seed germination, flower germination/fruit ripening.

Systematic Nomenclature Of Alkenes The general formula of alkenes: CnH2n – as

a result of double bond, alkene has 2 fewer H than alkane.

The general formula for cyclic alkene: CnH2n-2.

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

IUPAC rules for naming alkenes

1. Longest continuous chain containing the functional group:

- The functional group is numbered in the direction that gives the suffix the lowest possible no.

2. For compound with 2 double bonds, the suffix is ‘diene’

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

Note: When there are both a functional group and a substituent, the functional group gets the lowest number

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

5. If counting in either direction results in the same no for the alkene functional group, Name with the lowest functional group number and then the lowest substituent numbers:

6. A number is not needed to denote the position of the double bond in a cyclic alkene- the ring is always numbered so that the double bond is between C1 and C2.

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

Special Nomenclatures

The sp2 carbons of alkene called vinylic carbons sp3 carbon that is adjacent to vinylic carbon called an allylic

carbon.

Two groups containing a C-C double bond are used in common names- the vinyl group and allyl group.

Vinyl group- smallest group containing vinylic carbon Allyl group- smallest group containing allyllic C.

When ‘vinyl’ or ‘allyl’ is used, the substituent must be attached to the vinylic or allylic C.

A hydrogen bonded to a vinylic C- vinylic H A hydrogen bonded to a allylic C- allylic H

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

Which of the following is an allylic alcohol?

A) CH2=CHCH2OCH3

B) CH2=CHCH2CH3

C) HOCH=CHCH2CH3

D) CH3CH=CHCH2OH

E) CH2=CHCH2CH2OH

The Structure Of Alkenes

• Each double bond of an alkene has 3 sp2 orbitals overlaps an orbital of another atom to form a bond. Thus one of the C-C bonds in a double bond is a σ bond.

• The 2nd C-C bond in the double bond (the π bond) is formed from side-to-side overlap of the remaining p orbital of 1 C with sp2 C with the remaining p orbital of the other sp2 C.

Since 3 points determine a plane, each sp2 C and the two atoms singly bonded to it lie in a plane.

(a plane-defined as sp2 hybridized carbon is bonded to 3 atoms)

In order to achieve max orbital-orbital overlap, the 2 p orbitals must be parallel.

Therefore, all 6 atoms of the double bond system are in the same plane.

Alkenes Can Have Cis And Trans Isomers To achieve max overlap- 2 p orbitals forming the π

bond must be parallel. Therefore rotation about double bond does not readily

occur. If rotation to occur, the 2 p orbitals would cease to overlap and the π bond would break. the energy barrier to rotation about a double bond is high.

Because of the high energy barrier to rotation about C-C double bond, an alkene such as 2-butene can exist in 2 distinct form. The H bonded to the sp2 C can be on the same side of double bond or on opposite side of double bond.

Compound with Hs on the same side of double bond- cis isomer

Hs on the opposite side of double bond- trans isomer Cis isomer- substituents on the same side of the ring Trans isomer- substituents on opposite sides of the ring.

If one of the sp2 Cs of the double bond is attached to 2 identical substituents, there is only 1 possible structure for the alkenes.

Eg: CH3CH2C=CHCH3

CH2CH3

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 and different dipole moments

Naming alkenes using the E,Z system as long as each of the sp2 carbon of an alkene is

bonded to only 1 substituent, we can use the term cis and trans to designate the structure of alkenes

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

The E,Z system of nomenclature for alkenes with 2 substituents on 1 or both of the sp2 carbons.

To name an isomer by E, Z system- determine the relative priorities of the 2 groups bonded to 1 sp2 C

Then, the relative priorities of the 2 groups bonded to the other sp2 carbon.

E,Z is from German:

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

The relative priorities of the 2 groups bonded to a sp2 carbon are determined using the following rules:

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

2. If the 2 groups bonded to sp2 C start with the same atom, then move outward from the point of attachment and consider the atomic no of the atoms that are attached to the ‘tied’ atoms.

- The C of the CH2Cl group is bonded to Cl, H, H and the C of the CH2CH2Cl group is bonded to C, H, H.

- Cl has a greater atomic number than C, so CH2Cl group has higher priority.

- In CH2OH group and CH(CH3)2 group, both atoms are bonded C.

- The C of CH2OH group is bonded to O, H and H and the C for CH(CH3)2 group is bonded to C, C and H.

- Of these 6 atoms, O has the greatest atomic number so CH2OH has a higher priority.

3. Multiple bonds are treated as attachment of multiple single bonds using “divide-duplicate.”

- If an atom is doubly bonded to another atom, the priority system treats it as if it were singly bonded to two of those atoms. If an atom is triply bonded to another atom, the priority system treats it as if it were singly bonded to 3 of those atoms

- because the Cs immediately bonded to the sp2 carbon on the left are both bonded to C, H and H- ignore them-look at the groups that attached to them.

- 1 of the group is CH2OH and the other is C CH.- 1 C is bonded to H, H and O; the triple bonded C is considered to

be bonded to C, C and C. of the 6 atoms, O has the greatest atomic number, so CH2OH has higher priority.

At the other sp2 C- both atoms are Cs.

The 1st carbon of the CH2CH3 group is bonded to C, H and H.

1st C of the CH=CH2 is bonded to an H and doubly bonded to a C, so it is considered to be bonded to H, C and C.

1 C cancels in each groups (cancel atoms that are identical in the 2 groups), leaving H and H in CH2CH3 group and H and C in the CH=CH2. C has greater atomic no than H, so CH=CH2 has greater priority.

4. If 2 isotopes (atoms with same atomic number but different mass no) are compared, mass number is used to determine priorities.

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

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

exercises

Provide the proper IUPAC name for the alkene shown below.

a)

b)

(E)-3,4-dimethyl-3-heptene

(Z)-4-ethyl-3-methylheptene