Chap 14A - Alkenes

Describe nonemclature of alkenes

1. Identity longest continuous C chain containing C=C bond 

2. Identify the position of the C=C bond

• Number the carbon atoms consecutively from the end of the parent chain which gives the lower number for the position of the C=C bond

3. Identify the alkyl substituent(s) (CnH2n+1) attached to the parent chain and indicate the position(s) of the substituent(s)

• Arrange the substituents in alphabetical order, ignoring the prefixes such as di– or tri–

State name of this structure

Describe test for alkenes (reagents, conditions)

• Reagent and condition: Br2 in CCl4 or Br2 in H2O (aqueous Br2)

• Observation: Decolourisation of a solution of orange–red Br2 in CCl4 or orange Br2 in H2O

A good test for an organic compound should give an observable visual result where

there is: 

1. Colour change

2. Precipitate formed and the colour of the precipitate

3. Gas given off and identify the gas

Describe preparation of alkenes from dehydration of alcohols (type of reaction, reagents, conditions)

• Type of reaction: Elimination

• OH group and H atom from 2 adjacent C atoms are eliminated -> C=C bond formed with H2O as by-product 

• Reagents & conditions: concentrated H3PO4 catalyst + heat, excess concentrated H2SO4, heat OR Al2O3 + heat

Describe preparation of alkenes from Dehydrohalogenation of Halogenoalkanes (type of reaction, reagents, conditions)

• Type of reaction: Elimination

• Reagents & conditions: NaOH in ethanol + heat

• This is an acid-base reaction

Describe the products from Dehydrohalogenation of Halogenoalkanes (explain why one is the major product)

• When an unsymmetrical halogenoalkane (Eg. (CH3)2CHCH(Br)(CH3) heats with NaOH in ethanol, there are two products formed) 

• The major product formed is the more stable and substituted product 

  • The more substituted alkene is the one with more alkyl groups (CnH2n+1) bonded to the C atoms in the C=C bond

  • The enthalpy change of hydrogenation of the more substituted alkene to form the corresponding alkane is found to be less exothermic than its isomeric alkene which is less substituted

Describe solubility and density of alkenes

Solubility and Density

Insoluble in polar solvents such as water but soluble in non–polar solvents such as CCl4 Alkenes float on water and thus are less dense than water

Describe mp/bp of alkenes (number of C atoms and branched VS straight)

1. Number of C atoms 

• Boiling point increases with number of carbon atoms as more energy is required to overcome the stronger id-id interactions due to increasing number of electrons

2. Branched VS straight 

• Branched chain isomers have lower boiling points than their straight chain isomers

• Branched chain isomers are more spherical with less surface area of contact between molecules for electron interactions -> weaker id-id interactions

Describe mp/bp of alkenes (cis-trans isomer)

• Cis isomer has larger net dipole moment -> it is a polar molecule and is held by stronger pd-pd interactions and has a higher boiling point than the non–polar trans isomer 

  • Since the dipole moments in the trans-isomer are equal and opposite in direction, the dipole moments cancel out and net dipole moment becomes zero

  • In the liquid state, the molecules are not rigidly held in fixed positions and they are not very close together as in the solid state -> packing of molecules in trans isomer is not influential to account for differences in boiling points

• Trans isomer has a higher melting point than the cis isomer due to the closer packing of the trans isomer molecules

Explain why in C=C, strong sigma and weak pi

1. Strong sigma bond and weak pi bond 

• Since the π electrons are much more exposed than those in the σ bond, this makes the π bond a good source of electrons and the C=C bond is also a region of high electron density -> alkenes are more vulnerable to be attacked by electrophiles

• Typical reaction of alkenes is electrophilic addition

Describe addition of hydrogen halides (general reaction, type of reaction, reagents, conditions)

1. Addition of hydrogen halides 

• Type of reaction: Electrophilic addition

• Reagent and conditions: HX(g)

Describe mechanism of addition of hydrogen halides for symmetrical alkenes

For symmetrical alkenes

Only 1 product is formed  Formation of carbocation by electrophilic attack of Hδ+ from the polar Hδ+–Xδ– molecule on the C=C bond Heterolytic fission of pi bond in C=C and H-X bond  Attack by X- on carbocation

Describe addition of hydrogen halides for unsymmetrical alkenes (mechanism for propene)

For unsymmetrical alkenes

A mixture of two products is possible

Use marknovinikovs rule to identity major product formed from addition of hydrogen halides for unsymmetrical alkenes

Marknovnikov’s Rule

• Used to identity which is the major product 

• The major product of the electrophilic addition reaction will be the one formed from the more stable carbocation intermediate

• Stability of carbocation: tertiary > secondary > primary

• Tertiary carbocation is the most stable as it has three electron donating alkyl (R) groups, which exert electron donating inductive effect to help disperse the positive charge on carbocation to a greater extent