Alkanes- alkenes and stereoisomers

what is the general formula of alkanes?

CnH2n+2

are alkanes saturated or unsaturated?

saturated

what structure do the covalent bonds around each carbon atom of alkanes form?

• a tetrahedral structure

what is the name of the covalent bond within alkanes?

• sigma bonds

what are sigma bonds and can they rotate?

overlap of orbitals directly between the bonding atoms, free rotation of the σ-bond

why are alkanes non polar?

because their carbon and hydrogen atoms have similar electronegativities

are alkanes soluble or insoluble within water?

they are insoluble within water

why are alkanes insoluble in water?

• they have no permanent dipoles so cannot form hydrogen bonds

why are alkanes generally unreactive?

because of the strong covalent bonds within alkane molecules

what does a longer carbon chain result in?

• the stronger the induced dipole dipole interactions

• as there is more surface contact

• more energy needed to overcome induced dipole dipole interactions

why do branched chain alkanes have lower boiling points than their straight chain isomers?

• they have smaller molecular surface areas so induced dipole dipole forces are reduces

what type or reaction is the burning of alkanes?

• a combustion reactipn

why must liquid alkanes be vaporised first before they undergo combustion?

• combustion reactions happen between gases

• so liquid alkanes must be vaporised first

• smaller alkanes are more volatile so they burn more easier

define volatile?

turns into a gas more easily

why do larger alkanes make good fuels?

• larger alkanes release heaps more energy per mole as they have more bonds to react

what happens when alkanes react fully with oxygen?

• they undergo complete combustion to produce carbon dioxide and water

what happens when alkanes react partially with oxygen?

• they undergo incomplete combustion producing carbon monoxide and water or carbon particulates and water

what can carbon monoxide (CO) bind to, then prevent?

• it binds to haemoglobin before oxygen therefore prevents oxygen from being carried around the body

what is breaking covalent bonds called?

• bond fission

state the 2 ways in which covalent bonds can break?

• homolytic fission

• heterolytic fission

what occurs in heterolytic fission?

• the bond breaks unevenly with one of the bonding atoms recieving both electrons from the bonded pair

• two different substances are formed, a positively charged anion and a negatively charged cation

what occurs in homolytic fission?

• the bond breaks evenly and each bonding atom recieves one electron from the bonded pair

• 2 electrically uncharged radicals are formed

define radical?

• particles that have an unpaired electron

why are radicals very reactive?

• because of their unpaired electron

in what reactions do halogens react with alkanes, using what light?

in photochemical reactions using ultraviolet light

what occurs in a free radical substitution reaction?

• a hydrogen atom is reolaced by chlorine or bromine

what are the 3 reaction mechanism stages?

• initiation reactions

• propagation reactions

• termination reactions

state the steps of initiation?

• uv light provides enough energy to break the covalent bond

• homolytic fission occurs

• therefore the atom becomes a highly reactive free radical due to its unpaired electron

state the steps of propagation?

• a bromine radical reacts with a C-H bond in the alkane

• this forms an alkyl radical and a molecule of hydrogen bromide HBr

• each alkyl reacts with another bromine molecule

• this forms a halogenalkane and a new bromine radical

state the steps in termination?

• when two free radicals join together they make a stable molecule

• there are any possible termination reactions

what is the general formula of alkenes?

C nH2n

are alkenes saturated or unsaturated?

unsaturated

what 2 bonds is a double bond made from?

• a sigma and a pi bond

when is a sigma bond formed?

• when 2 s orbitals overlap in a straight line

when is a pi bond formed?

• by the sideways overlap of two adjacent p orbitals

why are pi bonds weaker than sigma bonds?

• because the electron density is spread about above and below the nuclei so the electrostatic attraction between the nuclei and the shared pair of electrons is weaker

do pi or sigma bonds have relatively low enthalpy?

pi bonds’

what is the shape around the c=c bond in alkenes?

trigonal planar

can c=c double bonds rotate?

no

define stereoisomers?

• compounds with the same structural formula but with a different arrangement in space

what are the 2 types of stereoisomers?

E and Z

how are groups positioned in a Z isomer?

has both groups has the same groups either above or below the double bond

how are groups positioned in an E isomer?

has the same groups positioned across the double bond

• if the groups of the higher priority are on the same side of the double bond what is the name of the isomer?

• if the groups of the higher priority are on the same side of the double bond the compound is the z isomer

• if the groups of higher priority are diagonally placed across the double bond what is the name of the isomer?

• if the groups of higher priority are diagonally placed across the double bond the compound is the E isomer

what other isomers can E/Z isomers also be called?

• cis- trans isomers

define the cis isomer?

has the hydrogen atoms on the same side

define the trans isomer?

has the hydrogen atoms on opposite sides

define electrophile?

electron pair acceptors

what do alkenes react with hydrogen to form?

alkanes

state the conditions needed for alkenes reacting with hydrogen to form alkanes?

Nickel catalyst , hydrogen and high temperatures

what do halogens react with alkenes to form?

dihaloalkanes

what is the name of the reaction when halogens react with alkenes to form dihaloalkanes?

electrophilic addition

what do alkenes react with hydrogen halides to form?

haloalkanes

what can alkenes be hydrated with steam to form?

alcohols

what conditions are needed for alkenes to react with steam to form alcohols?

• high temperatures

• high pressures

• acid catalyst, H3PO4

what can be used to test for alkenes?

bromine water, turns colourless in the presence of alkenes

what is the general formula of alcohols?

CnH2n+1OH

what are the types of alcohols?

• primary

• secondary

• tertiary

what makes alcohols polar?

the electronegative hydroxyl group which pulls the electrons in the C-OH bond away from the carbon atom

as alcohols get larger, does solubility increase or decrease ?

their solubility in water decreases

why does solubility decrease when alcohols increase in size?

in larger alcohols most of the molecule is a non polar carbon chain so there’s less attraction for the polar H2O molecules

what bonds do alcohols form with each other?

hydrogen bonds

why do alcohols have low volatility?

due to their strong hydrogen bonds

in substitution reactions involving alcohols what is the hydroxyl group replaced by, transforming the alcohol into what?

the hydroxyl group is replaced by the halide so the alcohol is transformed into a haloalkane

what takes place in eleimintaion reactions?

water is eliminated from alcohols

what is the oxidising agent which can oxidise alcohols?

acidified dichromate

Cr2O7 2–/H+ (i.e. K2Cr2O7/H2SO

what can primary alcohols be oxidised to form?

• aldehydes and then to carboxyllic acids

what can secondary alcohols be oxidised to form?

ketones

what can tertiary alcohols be oxidised to form?

tertiary alcohols cannot be oxidised

under what conditions will a primary alcohol completely oxidise to form a carboxylic acid?

when heated strongly under reflux with excess acidified dichromate

what is the general formula of aldehydes and ketones?

CnH2nO

what functional group do aldehydes and ketones have?

C=O

what do aldehydes have attached to their carbonyl carbon atom?

• a hydrogen and one alkyl group

what do ketones have attached to their carbonyl carbon atom?

• 2 alkyl groups

are electrophiles positively or negatively charged ions?

positively charged ions

define addition polymerisation?

• when the double bonds in alkenes open up and join together to make long chains called polymers

why are many polymers difficult to dispose of?

they are not biodegradable

where can waste plastics be buried?

in landfills

why is there a need to reduce landfill?

• because the amount of waste we generate is increasing there is a need to reduce landfills a much as possible

state 2 ways in which plastics derived from non renewable oil fractions can be reused?

• can be recycled be melting and remoulding them

• can be cracked into monomers and used as organic feedstock to make more plastics or other chemicals

why should we be cautious when burning waste plastics?

• polymers that contain chlorine e.g PVC produce HCL when burned which needs to be removed

define biodegradable polymers?

polymers which naturally decompose

what can biodegradable polymers be made from?

renewable raw materials such as starch or oil fractions

define photodegradable polymers?

polymers which decompose when exposed to light

define haloalknes?

alkanes with at least one halogen atom in place of a hydrogen

is the carbon halogen bond in haloalkanes polar?

yes

why is the carbon halogen bond in haloalkanes polar?

because halogens are more electronegative than carbon

define a nucleophile?

• an electron pair donor, negative ions

what is the name of the reaction when halogens are hydrolysed to alchohols?

• nucleophilic substitution reaction

what are the conditions needed to hydrolyse a haloalkane to an alcohol?

• a warm aqueous alkali

• reflux

what does the speed at which different haloalkanes are hydrolysed depend on?

bond enthalpy

which haloalkanes have the weakest bonds therefore hydrolyse quickest?

iodoalkanes

which haloalkanes have the strongest bonds therefore hydrolyse slowest?

fluoroalkanes