Module 4 (Core Organic Chemistry) Flashcards

Question

Reaction of Hydrogen Bromide with Alkenes:

Answer 1

Change in the functional group: alkene->haloalkane Reagent: HCl or HBr Conditions: Room temperature Mechanism: Electrophilic Addition Type of reagent: Electrophile, Hδ+ HBr is a polar molecule because Br is more electronegative than H. The H δ + is attracted to the electron-rich pi bond. This reaction can lead to two products when the alkene is unsymmetrical ‘Markownikoff’s Rule’ In most cases, bromine will be added to the carbon with the fewest hydrogens attached to it If the alkene is unsymmetrical, the addition of hydrogen bromide can lead to two isomeric products. Minor Product = 10% MajorProduct = 90% WHY? This carbocation intermediate is more stable because the methyl groups on either side of the positive carbon are electron-releasing and reduce the charge on the ion which stabilises it. The order of stability for carbocations is tertiary > secondary >primary In electrophilic addition to alkenes, the major product is formed via the more stable carbocation intermediate.

Answer 2

Industrially alkenes are converted to alcohols in one step. They are reacted with steam in the presence of an acid catalyst. CH2=CH2 (g) + H2O (g) -> CH3CH2OH (l) This reaction can be called hydration: a reaction where water is added to a molecule Reagent: steam Essential Conditions High temperature 300 to 600°C High pressure 70 atm Catalyst of concentrated H3PO4 The high pressures needed mean this cannot be done in the laboratory. It is preferred industrially, however, as there are no waste products and so has a high atom economy. It would also mean separation of products is easier (and cheaper) to carry out

Answer 3

Addition polymers are formed from alkenes Poly(alkenes) like alkanes are unreactive due to the strong C-C and C-H bonds The industrial importance of alkenes: The formation of polymers from ethene-based monomers is a major use of alkenes. The manufacture of margarine by catalytic hydrogenation of unsaturated vegetable oils using hydrogen and a nickel catalyst is another important industrial process. Liquid vegetable oils are generally polyunsaturated alkenes. Hydrogenation by the reaction of hydrogen using a nickel catalyst converts the double bonds to saturated single bonds. This increases the melting point of the oil making it harder and more solid. Dealing with waste polymers: Waste polymers can be processed in several ways. Separation and recycling The waste is sorted into different types of polymer (ie PTFE, PVC, PET) and then each type can be recycled by melting and remoulding. Combustion for energy production Waste polymers can be incinerated and the heat released can be used to generate electricity. Combustion of halogenated plastics (ie PVC) can lead to the formation of toxic, acidic waste products such as HCl. Chemists can minimise the environmental damage of this by removing the HCl fumes formed from the combustion process. Feedstock for Cracking Waste polymers can be used as a feedstock for the cracking process allowing for new production of plastics and other chemicals. Chemists have also been developing a range of biodegradable polymers, compostable polymers, soluble polymers and photodegradable polymers. Polymers formed from isoprene (2-methyl-1,3- butadiene), maize and starch are biodegradable

Answer 4

General formula alcohols CnH2n+1OH The alcohols have relatively low volatility due to their ability to form a hydrogen bond between alcohol molecules. The smaller alcohols (up to 3 carbons) are soluble in water because they can form hydrogen bonds with water. The longer the hydrocarbon chain the less soluble the alcohol. Uses of alcohols Ethanol is ‘alcohol’ in alcoholic drinks. Ethanol is commonly used as a solvent in the form of methylated spirits. Methanol is used as a petrol additive to improve combustion and is increasingly important as a feedstock in the production of organic chemicals;

Answer 5

Complete Combustion CH3CH2OH (l) + 3 O2(g) -> 2CO2(g) + 3 H2O(l) The products of complete combustion are CO2 and H2O. In excess oxygen, alcohols will burn with complete combustion.

Answer 6

Potassium dichromate K2Cr2O7 is an oxidising agent that causes alcohols to oxidise. The exact reaction, however, depends on the type of alcohol, i.e. whether it is primary, secondary, or tertiary, and on the conditions. Partial Oxidation: Reaction: primary alcohol -> aldehyde Reagent: potassium dichromate (VI) solution and dilute sulfuric acid. Conditions: (use a limited amount of dichromate) Warm gently and distil out the aldehyde as it forms: Observation: the orange dichromate ion (Cr2O7 2-) reduces to the green Cr 3+ ion Full Oxidation of Primary Alcohols: Reaction: primary alcohol -> carboxylic acid Reagent: potassium dichromate(VI) solution and dilute sulfuric acid Conditions: use an excess of dichromate, and heat under reflux: (distil off the product after the reaction has finished) Observation: the orange dichromate ion (Cr2O7 2-) reduces to the green Cr 3+ ion

Answer 7

Reflux: Reflux is used when heating organic reaction mixtures for long periods. The condenser prevents organic vapours from escaping by condensing them back to liquids.

Answer 8

Reaction: secondary alcohol -> ketone Reagent: potassium dichromate(VI) solution and dilute sulfuric acid. Conditions: heat under reflux Dehydration reaction: removal of a water molecule from a molecule

Answer 9

A mixture of halide ions with concentrated acid NaCl + H2SO4 can used for substituting a halogen for an alcohol CH3CH2OH + HCl -> CH3CH2Cl + H2O Various other halogenating compounds can be used to substitute the –OH group for a halogen NaCl + H2SO4 -> NaHSO4 + HCl Reaction: Alcohol -> Haloalkane Reagents: Concentrated sulfuric and sodium halide