12 - Alkanes Flashcards
Define alkanes
saturated hydrocarbons
General formula of alkanes
CnH2n+2
What do alkanes mainly make up
the main components of crude oil and natural gas.
What are alkanes mainly used for
Fuels
What is the bonding on alkanes
• Each carbon atom has 4 sigma (σ) bonds, either C-C or C-H.
What is each sigma bond a result of
the head-on overlap of two S orbitals – one from each bonding atom.
How does the sigma bond contain 2 electrons
Each orbital contains one electron, so the σ bond contains 2 shared electrons (directly between the bonding atoms).
Are sigma bonds in alkanes polar or non polar
Non polar
Why are sigma bonds in alkanes non polar
• The electronegativities of carbon and hydrogen are very similar
Result of non -polar sigma bonds in alkanes
alkanes are nonpolar molecules and have no partial positive or negative charges
What does alkanes being non - polar mean for its reactivity
• Alkanes therefore do not react with polar reagents
o They have no electron-deficient areas to attract nucleophiles
o And lack electron-rich areas to attract electrophiles
Shape of alkanes
• Each carbon atom has four valence electrons and will form four sigma bonds in an alkane
o Therefore there are no lone pairs present
o The shape will be tetrahedral around each carbon atom
Are the shapes of alkanes rigid
• NOO - σ-bonds act as axes which the atoms can rotate freely – the shapes are not rigid.
What is fractional distillation used for
separate crude oils into fractions using fractional distillation
Why does fractional distillation work
because the fractions have different boiling points
What is boiling points effected by
• Chain length + Branching
Effect of chain length on boiling points
• Increase chain length = increase surface area of contact = increase number/strength London forces between molecules = More energy required to overcome forces
Effect of branching on boiling point
• Increase branching = Less surface area of contact = Less intermolecular forces (London forces) = less energy needed to overcome London forces = lower boiling point
• More Branching = decreases boiling point
Does long chain length increase or decrease boiling point
Increase
Does branching increase or decrease boiling point
decreases boiling point
Do alkanes react with most common Reagans
NO
Why don’t alkanes react with common Reagents
because
1. C-C and C-H σ bonds are strong, difficult to break
2. C-C bonds are non-polar
3. Little/no difference in electronegativity between C and H non-polar
How are alkanes used in combustion
used as fuels
Why are alkanes used as fuels
because they are readily available and easy to transport
Can alkenes undergo complete and incomplete combustion
YES
How does complete combustion occur with alkanes
• Occurs in a plentiful supply of oxygen
Products of complete combustion of alkanes
• Products always carbon dioxide and water
How does incomplete combustion occur with alkanes
• Occurs in a limited supply a oxygen
• H atoms always oxidized to water
• Combustion of C is incomplete
Products of incomplete combustion
o CO forms (toxic)
o C forms (soot)
How does halogenation occur
via a 3 step mechanism = radical substitution
Radical substitution = mechanism in full
What happens in initiation step of radical substitution
What happens in propagation step of radical substitution
What happens in termination step of radical substitution
Draw the radical substitution mechanism for the halogenation of methane with bromine
Limitations of radical substitution
o 1. Further substitution
o 2. Substitution at different points in the carbon chain
Further substitution - limitation of radical substitution
Limitation - substitution at different points
