Topic 6 (Organic I)

Formation of halogenoalkanes from halogens and alkanes is an example of what kind of reaction?

Radical substitution reaction. Homolytic fission

What is the initiation step of halogenoalkane fission?

Cl₂ —UV—> 2Cl .

What are the propagation steps of halogenoalkane fission?

Cl . + CH₄ → . CH₃ + HCl

. CH₃ + Cl₂ → CH₃Cl + Cl .

What are the termination steps of halogenoalkane fission?

.Cl + .Cl → Cl₂

.CH₃ + .CH₃ → C₂H₆

.CH₃ + .Cl → CH₃Cl

What causes di,tri and tetrachloromethane to be produced? How can this be reduced? What is another issue with radical substitution?

Excess chlorine reacts with chloromethane to form other products. Use an excess of methane instead. Can take place anywhere along the carbon chain so isomers can be produce. (not with methane)

What is the propagation step for tetrachloromethane?

CHCl₃ + Cl . → CCl₃. + HCl

CCl₃. + Cl₂ → CCl₄ + Cl .

What is a radical?

Species with an unpaired electron.

What is cracking? What are the two types? Describe them.

The breakdown of larger chain hydrocarbons into smaller chain hydrocarbons to meet demand.

Thermal cracking

-High temp and pressure

-Produces alkenes to make polymers

Catalytic cracking

-Zeolite catalyst

-Produces aromatic hydrocarbons and motor fuels

-Cuts costs due to low pressure and temp, and saves time

What is reforming? Describe the process. Explain why it is done.

Conversion of straight chain alkanes into branched alkanes.

-use of platinum catalyst supported by aluminium oxide

-High temp

Prevents knocking (spontaneous explosion of alkanes in the fuel and air mixture of engines. Caused by straight chain alkanes) by reducing straight alkanes, increasing combustion efficiency by decreasing knocking possibility.

What kind of thermal reaction is combustion?

What are the products of complete and incomplete combustion?

Exothermic

Complete- Carbon dioxide and water

Incomplete- Carbon monoxide, Carbon and water, sometimes co2

What harmful pollutants are released by burning fossil fuels? How are they created and or how do they affect the environment?

Carbon monoxide -toxic, causes oxygen deprevation

Sulfur dioxide - caused by burning fossil fuels which contain sulfur

and nitrous oxides- caused by nitrogen and oxygen in air reacting in car engines

• cause acid rain which destroys wildlife

What can be used to decrease the amount of pollutants produced?

Catalytic converters found in car exhausts contain platinum which reacts with the harmful pollutants and makes then harmless, like water vapour or nitrogen, or less harmful like carbon dioxide.

2NO_(g) + 2CO_(g) → N_2(g) + 2CO_2(g)

Sigma bond

When orbitals overlap in a straight line in the space between atoms.

high electron density, electrostatic force and bond enthalpy- strongest bond

single bonds

Pi bond

Orbitals that overlap sideways

electron density spread out above and below nuclei- weaker electrostatic forces and bond enthalpy

found in double bonds

What kind of bond is in a double bond

Sigma and pi

What’s the difference between E/Z and Cis/Trans isomerism?

E/Z isomerism works when all groups bonded to the carbons are different, e.g 1-bromo,1-flouro,2-methylethene

Cis/Trans isomerism only works when the carbons share at least one group

1-bromo,2-methylethene

How are bromoalkanes formed from an alcohol?

-Red phosphorous, iodine and alcohol are refluxed, and PI3 is made in situ.

ROH + PI₃ → RI₃ + H₃PO₃

What are the two different ways chloroalkanes can be made from alcohol?

ROH + PCl₅ → RCl+ HCl + PCl₃

ROH + HCl → RCl + H₂O

What classification of alcohols are the most reactive?

Tertiary

How are bromoalkanes made from alcohol?

Alcohol + KBr + 50% conc sulfuric acid catalyst

-substitution reaction

How can alkenes be made from alcohol?

Dehydrated. Conc phosphoric acid catalyst. Hydroxyl group and H on adjacent carbon form water. This can create isomers

Test for aldehydes?

Benedict’s solution. Blue copper (II) ions (in sodium carbonate) are reduced to brick red copper (I) precipitate.

Fehling’s solution. Blue copper (II) ions (in sodium hydroxide) are reduced to form brick red precipitates

What is the oxidising agent used for alcohols? What is the colour change? What are the different kinds of alcohols oxidised to?

Orange acidified dichromate turns to green chromate ions when it oxidised primary alcohols to aldehydes then carboxylic acids and secondary alcohols to ketones. Tertiary alcohols cannot be oxidised further

How are aldehydes formed from the oxidisation?

-Excess primary alcohol with controlled amount of acidified dichromate solution in distillation apparatus.

-Aldehyde is collected quickly as its boiling point is lower than alcohols due to the double bonds…

How are carboxylic acids formed by oxidisation?

-Excess oxidising agent with aldehyde is heated under reflux

How are ketones formed from oxidation?

-Heat secondary alcohol under reflux with acidified dichromate.

Wha kind of reactions do alkenes (mainly) undergo?

Electrophilic addition

How to form alkanes from alkenes? Conditions?

Hydrogenation with hydrogen gas, nickel catalyst and 150 C

How is margarine made? Describe its properties

Hydrogenating unsaturated oils (double bonds). Solid at room temp and has a higher melting point than oil due to its stronger saturated bonds which require more energy to break.

Alkene + bromine = ? Describe

Dihalogenoalkane. Brown bromine water decolourises.

Alkene → alcohol?

Steam hydration (water) at 300C, 60-70atm, phosphoric acid catalyst

Oxidisation of alkenes

Purple acidified potassium manganate turns colourless. Oxidised to diols

Alkene → halogenoalkane. Problems

Hydrogen halides. Adding hydrogen halides to unsymmetrical alkenes can create more than one product. Product with the more stable carbocation is produced the most

What is the most stable kind of carbocation and why

Tertiary. All the alkyl groups feed electrons towards the positive charge

Halogenoalkanes → alcohols