6.2 alkenes, stereoisomerism & polymers Flashcards
give the general formula for an alkene.
CₙH₂ₙ
explain why alkenes are ‘unsaturated’ hydrocarbons.
- alkene molecules all contain at least one C=C bond.
- alkenes are unsaturated hydrocarbons because this C=C bond allows them to form bonds with other atoms in addition reactions.
explain when and why covalent bonds form in organic molecules.
- covalent bonds form when atomic orbitals from different atoms, each containing a single electron, overlap, causing the sharing of a pair of electrons.
- a covalent bond forms because the nuclei of the two atoms are electrostatically attracted to the bonding pair of electrons.
give the two types of covalent bond present in an organic molecule.
sigma (𝝈) bonds and pi (𝝅) bonds.
define the term ‘bond enthalpy’.
bond enthalpy refers to the amount of energy required to break a covalent bond in one mole of a molecule in a gaseous state.
explain why 𝝈 bonds have a high bond enthalpy.
- 𝝈 bonds form when two atomic orbitals overlap in a way that gives the highest electron density between the two positive nuclei.
- the high electron density between the nuclei causes a strong electrostatic attraction between the nuclei and the shared pair of electrons.
- as a result, 𝝈 bonds have a high bond enthalpy.
explain why 𝝅 bonds have a relatively low bond enthalpy as opposed to 𝝈 bonds.
- a 𝝅 bond is formed when the lobes of two atomic orbitals overlap sideways.
- in a 𝝅 bond, the electron density is spread out above and below the nuclei, which causes the electrostatic attraction between the nuclei and the shared pair of electrons to be weaker in a 𝝅 bond.
- as a result, 𝝅 bonds have a relatively low bond enthalpy compared to 𝝈 bonds.
explain the difference between the C-C bond present in alkanes and the C=C bond present in alkenes.
- in an alkane, the C-C bond contains only a 𝝈 bond.
- in an alkene, the C=C bond contains both a 𝝈 bond and a 𝝅 bond.
explain why carbon atoms in a C=C bond and the atoms bonded to these carbons form a trigonal planar shape.
- carbon atoms in a C=C double bond and the atoms bonded to these carbons lie in the same plane.
- the attachment of atoms to each double-bonded carbon forms a trigonal planar shape.
give the bond angle of the H-C-H bonds found in the planar unit.
120°
define the term ‘stereoisomer’, and describe when stereoisomerism occurs.
- stereoisomers are molecules which have the same structural formula but different spatial arrangement.
- stereoisomerism occurs when the two double-bonded carbon atoms each have two different atoms or functional groups attached to them.
give an example of stereoisomerism.
E/Z isomerism.
give the difference between the ‘Z-isomer’ and the ‘E-isomer’ formed as a result of E/Z isomerism.
- the Z-isomer has the same groups present either above or below the C=C double bond.
- the E-isomer has the same groups positioned across the C=C double bond.
give the rule used to work out the E/Z isomerism for any alkene.
the Cahn-Ingold-Prelog (CIP) rule.
if the carbon atoms have at least one group in common, the isomers formed will be either ‘cis’ or ‘trans’. give the difference between a cis isomer and a trans isomer.
- cis isomers have the same atom or functional group present on the same side of the C=C double bond.
- trans isomers have the same atom or functional group present on opposite sides of the C=C double bond.
give the name of the reaction which alkenes participate in.
electrophilic addition reactions.
define the term ‘electrophile’, and give two examples of electrophiles.
- electrophiles are electron-pair acceptors.
- electrophiles include positively charged ions, such as H⁺, and polar molecules.
explain why alkenes participate in electrophilic addition reactions.
- in an electrophilic addition reaction, the alkene double bond separates, and atoms are bonded to the carbon atoms.
- alkenes participate in electrophilic addition reactions because the C=C bond contains a lot of electrons, so is easily attacked by electrophiles.
ethene reacts with hydrogen gas in an electrophilic addition reaction to produce ethane. give the reagent and conditions required for this reaction of occur.
- a nickel catalyst.
- a temperature of 150°C.
what is a carbocation?
a carbocation is an organic ion containing a positively charged carbon atom.
halogens react with alkenes to form dihalogenoalkanes. give the reagent, and describe the test used to test for the presence of a C=C double bond.
- bromine water is used to test for the presence of a C=C double bond.
- when an alkene is shaken with bromine water, the solution quickly decolourises.
- this occurs because bromine is added across the C=C double bond to form a colourless dihalogenoalkane, dibromoalkane.
alcohols can be manufactured from alkenes via steam hydration. give the reagent and conditions required for this reaction to occur.
- a solid phosphoric (V) acid catalyst.
- H₂O (l)
- a temperature of 300°C.
- a pressure of 60 - 70 atm.
alkenes can be oxidised to produce diols (alcohols with two -OH groups present) give the reagent required for this reaction to occur, and the result of the reaction.
- reagent: acidified potassium manganate (VII)
- result: shaking an alkene with the acidified potassium manganate will cause the purple solution to decolourise.
adding hydrogen halides to unsymmetrical alkenes forms two products. the amount of each product formed depends on how stable the carbocation formed in the middle of the reaction is. explain why a secondary or tertiary carbocation is more likely to form than a primary carbocation.
- secondary and tertiary carbocations contain two and three alkyl groups respectively, as opposed to primary carbocations, which only contain one alkyl group.
- carbocations with more alkyl groups are more stable because the alkyl groups feed electrons towards the positive charge.
- therefore, a secondary or tertiary carbocation is more likely to form than a primary carbocation, due to having more stability.
