Chemistry OCR B (Salters) The Polymer Revolution

Alkenes are Unsaturated Hydrocarbons

 Examples being propene(CH2CHCH3) & penta-1,3-diene (CH2CHCHC2H5)

Adding Hydrogen to C=C Bonds Produces Alkanes

Ethene+Hydrogen->Ethane (H2C=CH2 + H2->[nickel, high pressure, 150 degrees/ platinum r.t.p] CH3CH3)

Use Bromine Water to Test for C=C Double Bonds

Bromine water + Cyclohexene -> [SHAKE] -> solution goes from a red/brown colour to colourless

Electrophillic Addition Opens Up the Double Bond

Double bond repels electrons in Br2, polarising Br-Br -> Heterolytic fission of Br2 , the closer Br gives up bonding electrons to other Br & sticks to the Carbon atom -> + charged carbocation intermediate -> Br- bonds to other Carbon.Addition reaction of alkenes with hydrogen halides are also undergone: Bromoalkanes formed in the reaction between ethene & HBr, C2H4 +HBr -> C2H5Br (Other alkenes react in a similar way)

Alcohols are Primary, Secondary or Tertiary

Primary alcohol -> Carbon attached to OH is attached to ONEalkyl groups. Secondary alcohol -> Carbon attached to OH is attached to TWOalkyl groups. Tertiary alcohol -> Attached to THREE.

Aldehydes and Ketones Both Contain a Double-Bonded Oxygen

Aldehydes have a C=O at the end of a carbon chain & Ketones have it anywhere except at the end.

Carboxylic Acids Contain a -COOH Functional Group :  Carboxylic acids are organic acids.

Alcohols can be Dehydrated to Form Alkenes:  CH2H5 -> CH2 + H2O

Oxidation of Alcohols:  Burning them/ using an oxidising agent. Tertiary alcohols won't be oxidised.

Reaction Conditions Decide Whether You Get an Aldehyde or Carboxylic Acid

Gently heat ethanol with potassium dichromate & sulfuric acid in test tube. To get just the aldehyde heat alcohol with oxidising agent in distilation apparatus. To get the carboxylic acid, the alcohol needs to be vigorously oxidised, mixed with excess oxidising agent & heated under reflux.

Hydrogen Bonding is the Strongest Intermolecular Force:  Hydrogen bonding only occures when hydrogen is covalently bonded to fluorine, nitrogen or oxygen. They are very electronegative, so draw bonding electrons away from the hydrogen atom. The bond is so polarised & hydrogen has such a high charge density, that the hydrogen atoms form weak bonds with lone pairs of electrons on F, N and O atoms of other molecules. H2O & NH3 have hydrogen bonding.

Hydrogen Bonds Explain Why Ice is Less Dense than Water:  As ice melts some hydrogen bonds are broken, allowing other molecules to fill the spaces, making water more dense than ice. Water is at its densest at about 4 Degrees Celsius.

Stronger Intermolecular Forces mean Higher Melting and Boiling Points:  To boil a liquid, you need to overcome the intermolecular forces. The higher the boiling point the stronger the intermolecular forces. substances that form hydrogen bonds have higher boiling & melting points.

Hydrogen Bonding Means Some Polymers Dissolve in Water:  A substance will dissolve if the molecules in the substance are able to form bondswith the water. Polymers that have -OH groups can form hydrogen bonds with water molecules. Polymer molecules can also bond to each other by hydrogen bonding if the polymer has loads or very few -OH groups. If this isn't the case the polymer will be soluble.

Alkenes Join up to form Addition Polymers:  Double bondsin alkenes can open up and join together to make long chains, called polymers. An example is ethene being made into poly(ethene) via addition polymerisation. Co-polymers are made up of more than one type of polymer- they join in a random order.

Monomers Will Always Have a Double Bond:  To work out the monomer, take the repeating unit and add a double bond. Because there is no double bond in poly(alkenes), there're unreactive.

Cross- Linking Affects how Polymers Behave when there're Heated

Thermoplastic polymers do not have cross-links, and are held together by weak intermolecular forces. As they are weak they are easy to overcome , so easy to melt. Thermosetting polymers, like bakelite, have covalent cross-links meaning they do not soften when heated. They are strong, hard & insoluble.

A Polymers Uses Depend on its Properties:  Poly(chloroethene) is durable and flexible. It has a wide range of uses and is used to make water pipes, insulationon electric wires and as building material. Poly(tetrafluoroethene) is chemically inert & has non-stick properties, so is ideal for making frying pans. Polystyrene/ poly(phenylethene) is cheap & can be made into expanded polystyrene, which is a good insulator.

Atoms Can't Rotate Around Double Bonds

Single bonds allow atoms to rotate freely, however atoms can't rotate around a C=C double bond. The restricted rotation around C=C double bonds is what cause E/Z isomerism.

E/Z isomerism is a Type of Stereoisomerism

Stereoisomerism have the same structural formula but a different arrangement. Because of the lack of rotation around the double bond, some alkenescan have stereoisomers. This happens when the two double bonded carbon atoms have different groups attached to them. When the same groups are across the double bond, then its the E-isomer. When the same groups are both above or below the double bond, then it is the Z-isomer.

E/Z Isomers Can Sometimes Be Called Cis- Trans Isomers

(i) 'cis' means the Z-isomer, and (ii) 'trans' means the E-isomer. So... E-but-2-ene can also be called trans-but-2-ene, and Z-but-2-ene can also be called cis-but-2-ene.

Infrared Spectroscopy Helps You Identify Organic Molecules

In IR spectroscopy, a beam of IR radiation is passed through a sample of a chemical. The IR radiation is absorbed by the covalent bonds in the molecules which makes the bond vibrate more. Bonds between different atoms absorb different frequencies of IR radiation, so the O-H in an alcohol and the O-H in a carboxylic acid absorb different frequencies.

You Need to be Able to Interpret an Infrared Spectrum

An IF spectrometer produces a graph that shows you what frequencies of radiation the molecules are absorbing. You can therefore use it to identify the functional groups in a molecule.

The Fingerprint Region Identifies a Molecule

A region between 1000cm-1 and 1550cm-1 on the spectrum is called the fingerprint region. It is unique to a particular compound. If the region corresponds to the known compounds then, hey presto- you know what the molecule is.