Alkenes Flashcards
What are alkenes?
Unsaturated hydrocarbons because of the presence of at least one carbon-carbon double bond
What is the general formula of alkenes?
CnH2n
How does the number of alkyl groups attached to C=C bond affect the stability of alkenes?
The more alkyl groups attached to C=C bond, the more substituted the alkene, the more stable the alkene
Is cis-isomer or the trans-isomer more stable and why?
Trans-isomer is more stable than the cis-isomer
- For cis-isomer: bulky groups are on the same side of the double bond, causing steric hindrance, thus less stable
- For trans-isomer: less steric hindrance as bulky groups separated on opposite sides of double bond, thus more stable
What happens to the boiling and melting point of alkenes as the number of C atom increases?
As the number of C atom increases, the number of electrons increases, more energy is required to overcome the stronger id-id between molecules, thus boiling and melting point of alkene increases
Do straight-chain alkene or branched isomer have higher boiling point and why?
Straight chain alkene has a larger surface area compared to the branched, more spherical alkene, thus more energy is required to overcome the more extensive id-id interactions between alkene molecules
Do cis-isomer or trans-isomer have a higher boiling point and why?
Cis-isomer typically have a higher boiling point because trans-isomer’s dipole moment will cancel out, forming weaker id-id interactions while cis-isomer will have net dipole moment forming stronger pd-pd
Do cis-isomer or trans-isomer have a lower melting point?
Cis-isomer has a lower symmetry, so the cis-isomer does not pack as well into a crystalline structure compared to the trans-isomer, poor packing of cis-isomer meant that intermolecular forces are not as extensive so less energy is required to melt the solid
What is the industrial preparation of alkene?
Cracking of crude oil
What are the laboratory preparation methods for alkenes?
- Elimination of H2O
- Elimination of HX
What is the reagents and conditions for cracking of crude oil?
Thermal cracking: 700-900 degree celsius
Catalytic cracking: Al2O3 + 500 degree celsius
What are the reagents and conditions for the elimination of H2O to form alkenes and what does the reagent act as?
- Pass alcohol vapour over heated Al2O3 (Al2O3 acts as a heterogeneous catalyst)
- Heat alcohol with excess conc. H2SO4 (conc. H2SO4 acts as a dehydrating agent)
What are the reagents and conditions for the elimination of HX to form alkenes?
ethanolic KOH + heat under reflux
What is the major product from the elimination reaction to form alkenes?
By Saytzeff rule, major product is more substituted alkene
Are alkenes more or less reactive than alkanes and why?
Alkenes are more reactive than alkanes because C=C bond is electron rich is vulnerable to attack by electrophiles
What reactions do alkenes undergo?
- combustion
- reduction
- electrophilic addition
- oxidation with KMnO4 (aq)
- addition polymerisation
What are the reagents and conditions for reduction reaction?
H2 (g), Ni/Pd/Pt catalyst, heat
What are the types of electrophilic addition alkenes can undergo?
- electrophilic addition of HX
- electrophilic addition of halogen
- electrophilic addition of steam
- electrophilic addition of conc. H2SO4, followed by heating with water
What is reagents and conditions for electrophilic addition of HX?
HX (g), room temperature
What is the order of reactivity of HX (from least to most reactive) and why?
HCl < HBr < HI
Bond strength:
E(H-Cl) > E(H-Br) > E(H-I)
What is the major product of electrophilic addition to unsymmetrical alkene and why?
X adds to the more substituted carbon
Alkyl groups are slightly electron-donating and help to reduce the positive charge on the carbocation, thereby stabilising the carbocation. Thus, the greater the number of alkyl groups attached to the electron-deficient carbon, the more stable the carbocation
What are the reagents and conditions for electrophilic addition of halogen?
- Cl2(g) or Br2(l), room temperature
- X2 dissolved in CCl4, room temperature
What is the observation of the electrophilic addition of halogen?
__(colour)__ (in what medium) will be decloursied [refer to data booklet for the colour]
What is the major product of the electrophilic addition of halogen in aqueous medium and why?
halohydrin because since water is the solvent, there will be much more H2O molecules than halide ions
