Alkanes and alkenes Flashcards
What is an alkane?
A homologous series of hydrocarbons with single bonds.
What is the shape of an alkane?
Tetrahedral, 109.5’
How is a sigma bond formed?
Overlap of 2 orbitals, 1 from each bonding atom. Positioned on line directly between bonding atoms.
How is crude oil separated?
By fractional distillation. Boiling point increases and chain length increases. London forces acting between molecules in close surface contact. The more contact points, stronger the London forces - boiling point increases.
How is the boiling point of isomers affected?
Different boiling point but same molecular mass as branching increases, less points of contact, so weaker London forces.
How do alkanes combust?
Complete - lots of oxygen present
Incomplete - not enough oxygen. Produces CO or C. CO is toxic as binds to Hb instead of O2 - oxygen deprivation.
Are alkanes reactive?
Not very, as stable molecules due to only having single bonds.
How do alkanes react with halogens?
With UV light for energy. Radical substitution reactions.
What are the steps for radical substitution?
- Initiation - produce free radicals
- Propagation - free radicals used up/created in chains reaction
- Termination - free radicals used up
What is an alkene?
Homologous series of hydrocarbons with 1 or more carbon double bond
How is the double bond formed?
Sigma bonds use up 3 electrons. 1 electron in the p orbital is left - a pi bond formed from the sideways overlap of 2 p orbitals. Density concentrated above and below the nuclei.
What are the properties of a pi bond?
Fixes the 2 carbons in place and stops rotation. Much weaker as have electron density spread out so low bond enthalpy.
What is the bond angle around the double bond?
120’ because 3 areas of electron density around each carbon.
What is stereoisomerism?
The same structural formula with a different arrangement of atoms in space.
Forms because the double bonds cant rotate.
What is E/Z isomerism?
Different groups attached to C=C.
E - same groups diagonally across c=c
Z - same groups across c=c.
What reactions do alkenes undergo?
Electrophilic addition reactions
How are alkenes hydrogenated?
H2, nickel catalyst, 150’C to form an alkane
How are alkenes halogenated?
Rapid reaction at room temperature
How are alkenes reacted with hydrogen halides?
Gaseous hydrogen halides at room temperature to form haloalkanes.
How are alkenes hydrated?
Form alcohols when reacted with steam, phosphoric acid catalyst at 300’C and 150 atm.
What is an electrophile?
An electron pair acceptor
What is the test for alkenes?
Decolorisation of bromine water as it adds across the double bond.
What is Markowninikoff’s rule?
When a hydrogen halide reacts with unsymmetrical alkene, the hydrogen attaches to the carbon atom with the most hydrogen atoms and the fewest carbon atoms.
What 2 products can form from a reaction with an alkene?
The major or minor product. Major is more likely to be formed as the carbocation is more stable.
