chapter 2- alkanes

properties of alkanes?

saturated hydrocarbons called sigma bonds

• the sigma bond is the result of the overlap between 2 orbitals, one from each bonding atom

• each overlapping orbital contains one electron, so therefore, the sigma bond has two electrons that are shared between the bonding atoms

• there is a free rotation around the sigma bond

properties of alkanes 2.0?

part of a homologous series with general formula : CnH2n+2

• only exception to this is cycloalkanes

properties of alkanes 3.0?

each carbon atom in an alkane forms 4 sigma bonds.

• they have a tetrahedral shape and a bond angle of 109.5

• 4 bonding pairs repel

• this shape minimises electron-electron repulsion between bonding pairs.

boiling points of the first ten alkanes?

• methane: -164

• ethane: -89

• propane: -42

• butane: -1

• pentane: 36

• hexane: 69

• heptane: 98

• octane: 126

• nonane: 151

• decane: 174

what acts between organic alkane chains?

induced dipole-dipole interactions

what are induced dipole-dipole interactions affected by?

the chain length and any branching

what causes stronger intermolecular forces between chains and therefore a higher boiling point?

as the chain length of alkane increases, so does the Mr of the molecule

what does branching of the alkane chains do to the van der Waals forces?

it weakens them as they are less able to pack together tightly

• more branching gives less surface contact

• therefore, the distance over which the intermolecular forces act is increases and the attractive forces are weakened

• this is why branched alkanes have a lower boiling point than straight alkane chains

what does branching do to the boiling points?

decreases them

• since there are fewer/weaker intermolecular forces, less energy is needed to break the london forces

why does boiling point of hydrocarbons increase with increasing molecular size?

• increasing number of electrons in the bigger molecules, causing an increase in the size of london forces between the molecules

what happens in fractional distillation to split up crude oil?

• the hydrocarbons are vaporised and fed into fractioning column

• vapours rise, cool and condense at different boiling points

• the column is hotter at the bottom and cooler at the top

• products with short carbon chains have lower boiling points, meaning they rise higher up the column before reaching their boiling point

• therefore, they are collected at the top of the column

• whereas, long chain hydrocarbons have higher boiling points, meaning they don’t rise up very far before reaching their boiling point

• they condense and are collected at the bottom of the fractioning column

why does fractional distillation work?

• because the hydrocarbon fractions have different boiling points

what is cracking?

• conversion of large hydrocarbons to smaller hydrocarbons by the breakage of C-C bonds

why is cracking done (discuss from economic point of view)?

• short chain fractions are in more demand

• long chain fractions larger in quantity can be used to make more short chain molecules

• cracking products more valuable than the starting material

why do alkanes have low reactivity?

due to the high bond enthalpy of the carbon to carbon single bond and a very low polarity of the sigma bond present

why does the boiling point increase?

• because of the weak intermolecular forces called london dispersion forces

• they hold molecules together, but once broken, the molecule move apart from each other and become a gas

• the greater the intermolecular forces, the higher the boiling point

why are alkanes used as fuels?

because they are readily available, easy to transport and burn in a plentiful supply of oxygen without releasing toxic products

harmful effect of carbon monoxide?

• can form a strong bond with haemoglobin in red blood cells

• this is a stronger bond than made with oxygen and therefore prevents oxygen from attaching to the haemoglobin

harmful effect of soot?

• can cause global dimming-reflection of the sun’s light

reactions of alkanes with halogens?

• in the presence of UV light, alkanes react with halogens

• this is called a substitution reaction

what is the mechanism for the bromination of methane an example of?

radical substitution

what are the 3 stages of radical substituiton?

initiation, propagation and termination

what happens in initiation?

• the reaction is started when the covalent bond in a bromine molecule is broken by homolytic fission

• each bromine atom takes one electron from the pair, forming 2 radicals

what happens in propagation?

• in the first propagation step, the bromine radical reacts with a C-H bond in the methane, forming a methyl radical and a molecule of hydrogen bromide, HBr

• in the second propagation step, each methyl radical reacts with another bromine molecule, forming the organic product bromoethane, and a new bromine radical

• this then continues in a chain reaction until all reactants have been used up

what happens in termination?

• 2 radicals collide, forming a molecule with all electrons paired

• there are a number of possible termination steps with different radicals in the reaction mixture

• when 2 radicals collide and react, both radicals are removed from the reaction mixture, stopping the reaction

name another bromine-containing organic product which is formed when methane reacts with bromine?

• dibromomethane

why does radical substitution produce a mixture of organic products?

• because different termination products are produced

• substitution occurs at different positions along the chain

limitations of radical substitution in synthesis by formation of a mixture of organic products?

• because different products are formed, reaction is not selective

• this gives a low yield of the desired product

• and requires difficult separation and purification, making it unsuitable for synthesis

why would long chain straight isomers be converted into branched chain isomers?

• branched chains are easier to combust