Alkenes Flashcards
What is the general formula of an alkene?
CnH2n
Why are alkenes described as ‘unsaturated’?
Alkenes contains at least 1 double bond.
What bonds is a double covalent bond made of ?
A sigma and pi bond.
Why are sigma bonds the strongest type of covalent bond?
Sigma bonds are when 2 S orbitals overlap and aligns horizontally. There is a strong electrostatic attraction between nuclei and shared electrons, due to a high electron density. This strong electrostatic attractions means more energy is needed to break the bond, giving a high bond enthalpy.
What makes a single covalent bond?
A sigma bond.
Why are pi bonds weaker than sigma bonds?
Electron density is spread out above and below the nuclei. Electrostatic attraction is lower, and this also gives a lower bond enthalpy.
Why are alkanes less reactive than alkenes?
Alkanes have sigma bonds, which have a high bond enthalpy, meaning more energy is needed to break this bond, hence is less reactive. Alkanes are also non-polar.
What is an electrophile?
An electron pair acceptor.
What is halogenation?
When a halogen molecule reacts with an alkene, to form a dihaloalkane.
How can haloalkanes be formed?
Reacting alkenes with hydrogen halides.
The formation of haloalkanes and dihaloalkanes occur by what mechanism?
Electrophilic addition mechanism.
In the formation of a dihaloalkane, how is a dipole initially caused in the halogen molecule? What type of bond fission is this?
The double bond in the alkene is electron rich as it shares 4 electrons. When the halogen molecule approaches the double bond, the double bond pushes the electrons within the halogen molecule (as electrons repel), inducing a dipole. Both electrons goes to one bonding atom in the halogen molecule, which is heterolytic bond fission. The bonding atom that has lost its shared electron is now delta positive and now acts as an electrophile.
How may we test for an alkene?
Testing an alkene with bromine water is also a halogenation reaction. If the bromine water turns from orange to colourless, there is an alkene present.
When a hydrogen halide reacts with an alkene, there is no need to induce a dipole on the hydrogen halide. Why is this?
The hydrogen halide bond is already polar, with the halogen being more electronegative.
When forming a haloalkane, why may there be a mixture of products?
The positions the hydrogen and halogen reacts with on the alkene can alternate.
In the formation of haloalkanes, there will be major and minor products from the mixture. How do we determine which is the major product?
The major product is the product which has more carbons bonded to the carbocation. If there is more carbons bonded to the carbocation (primary, secondary, tertiary), it is more stable.
What are the reagents (and their states) in a hydrogenation reaction? What are the products?
In a hydrogenation reaction, an alkene reacts with hydrogen gas. This forms an alkane.
State the conditions needed for a hydrogenation reaction.
150 degrees celcius and a nickel catalyst.
What covalent bond breaks in a hydrogenation reaction?
A pi bond.
What is the product of a hydration reaction?
An alcohol.
What conditions are needed for a hydration reaction?
Phosphoric acid catalyst.
Why may there be a mixture of products formed?
The positions the -OH and -H reacts on the alkene can alternate.
What is a polymer?
A long chain made of monomers.
State the conditions needed for a polymerisation reaction to occur?
High temperature, high pressure and a catalyst.
Why may storing polymers in landfill sites cause pollution?
Alkenes are non-biodegradable.
What are the advantages and disadvantages of burning polymers?
Burns them instead of a non-renewable source like fossil fuels. But burning polymers releases HCl gas which is toxic.
What are ways to dispose of polymers?
Recycling, feedstock recycling, burning, landfills.
