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.
