Chemistry (P)

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  • Created on: 28-05-13 16:45

Alkenes and Naming Alkenes

Alkenes = basic hydrocarbon units of many polymers.

Polymers can be made of one or more types of alkene monomer.

General formula for alkenes = CnH2n

Alkenes have one or more carbon-carbon (C=C) double bonds - for this reason they are said to be unsaturated.

Names of alkenes always end in -ene.

Number preceeding the -ene = the position of the C=C bond e.g. but-2-ene (CH3-CH2-CH=CH2).

Alkenes can be cyclic compounds.

Diene = alkene with 2 C=C bonds e.g. hexa-1,3-diene (note the addition of an 'a'after hex).

All bond angles around a C=C bond are 120 degrees (three groups of electrons around each carbon atom - two singles and one double).

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Reactions of Alkenes

Alkenes undergo electrophilic addition.

Example: Ethene.

There are four electrons in the double bond of ethene (give the region between the two carbon atoms a high negative charge density).

Electrophiles = either positive ions or molecules with a partial positive charge on one of the atoms, react and accept a pair of electrons.

Electrophiles are attracted to negatively charged region in the alkene - accept a pair of electrons from the double bond at the start of the reaction.

CH2=CH2 + X-Y --> CH2X-CH2Y

The mechanism for addition of bromine to ethene demonstrates the reaction mechanism for electrophilic addition.

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Addition of Bromine to Ethene

Electrophilic addition:

  • Bromine molecule becomes polarised when it approaches an alkene.
  • Electrons in the bromine molecule = repelled back along the molecule.
  • Electron density is now unequally distributed.
  • Bromine atom nearest to the alkene becomes slightly positively charged - now acts as an electrophile.
  • A pair of electrons from the C=C bond of the alkene moves towards the slightly charged bromine atom - a C-Br bond is formed.
  • Carbon species is now positively charged = carbocation.
  • The other bromine (which is now negatively charged) moves rapidly to make another bond.

Overall process is the addition by an electrophile across a double bond = Electrophilic addition.

Model for this mechanism is supported by experimental evidence.

If Cl- ions are present when ethene and bromine react then the intermediate carbocation reacts with both bromide and chloride ions, forming both dibromoethane and 1-bromo-2-chloroethane.

Bromine = test for unsaturation (becomes decolourised).

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Electrophiles Reacting with Alkenes

Electrophile: Product:  Conditions:

- Br2 - CH2BrCH2Br  - Room temperature and pressure.

(1,2-dibromoethane)

- Br2(aq) - CH2BrCH2OH - Room temperature and pressure

(2-bromoethanol) (water can attack the intermediate carbocation)

- HBr(aq) - CH3CH2Br - Aqueous solution, room temperature and pressure.

(bromoethane)

- H2O (H-OH) - CH3CH2OH - Phosphoric acid/silica at 300C/60atm or with conc. (ethanol) sulphuric acid then H2O at 1atm.

- H2 - CH3CH3  - Pt catalyst at room temperature and pressure or Ni (ethane) catalyst at 150C/5atm.

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Addition Polymerisation

Alkenes can undergo addition polymerisation.

Unsaturated starting molecules (monomers) join together to form a long chain saturated polymer with no other products.

e.g. CH2=CH2 + CH2=CH2 ---> -CH2-CH2-CH2-CH2

The right conditions are:

  • gaseous phase
  • high pressure
  • high temperature
  • catalyst

Polymer name is made by putting poly in front of the alkene's name in brackets e.g. poly(ethene).

The polymer is NOT an alkene as it is saturated.

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E/Z Isomerisation

E/Z Isomerisation = type of stereoisomerism.

Stereoisomerism = atoms bonded in the same order but arranged differently in the space inside each each isomer.

Cis-trans = for simple molecules, E/Z = for simple and complex molecule.

The reason why two isomers exist is because one of the bonds in a C=C double bond needs breaking to turn one isomer into the other.

There is not enough energy for this at room temp.

  • Isomer has groups on the same side = Cis or Z isomer e.g. cis-but-2-ene or (Z)-but-2-ene.
  • Isomer has groups on opposite sides = Trans or E isomer e.g. (E)-but-2-ene.
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Addition Polymerisation and Co-Polymerisation

Co-polymerisation

Polymer = long molecule made from lots of smaller molecules (monomers).

A co-polymer is made when two different polymers are incorporated into the polymer chain.

e.g. A + B + A + B ---> -A-B-A-B-

Addition Polymerisation

If monomers contain a double bond they can add together to make a polymer.

This is addition polymerisation - no other product is formed.

Polymer structure can be represented by drawing the repeating unit.

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Properties of Polymers

  • Chain length - longer chains = stronger polymer. Tensile strength = a measure of how much force needs to be applied befor a polymer snaps. Tensile strength increases because A) Longer chains become more entangled - B) Longer chains = stronger intermolecular bonds so more difficult to pull apart.
  • Side groups on the polymer chain - more polar the side groups = stronger the bonds between polymer chains = stronger polymer.
  • Branching = straight, unbranched polymer chains pack together closer - allows stronger bonds between chains.
  • Chain flexibility - more rigid the chain = stronger polymer. Hydrocarbon chains = very flexible, adding benzene rings makes the polymer stiffer.
  • Cross-linking = more extensive cross linking = harder to melt polymer.
  • Stereoregularity = more regular the orientation of the side groups, the closer the packing = stronger polymer.

Elastomers = soft springy polymers - can be stretched but return to their original shape e.g. rubber.

Plastics = polymers that are easily moulded e.g. poly(ethene).

Fibres = polymers which can be made into strong thin threads e.g. nylon and poly(propene).

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Thermoplastics and Thermosets

Thermoplastics

Polymers without cross-links between chains = thermoplastics e.g. poly(ethene).

Intermolecular bonds are much weaker than the covalent bonds in thermosets.

Attractive forces in thermoplastics can be overcome by warming - chains slide over each other = polymer becomes deformed (changes shape)

On cooling, weak bonds between polymer chains reform and a new shape if formed.

Thermosets

E.g. Bakelite. These have extensive cross-linking between different polymer chains - much stronger than in thermoplastics.

Covalent bonds can't be broken by warming - chains can't move relative to one another so polymers can't change shape.

Heating just chars and burns the polymer.

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Hydrogen Bonding

Hydrogen bonds = much stronger than other types of intermolecular bonds.

  • There is a large dipole between small hydrogen atom and a highly electronegative atom e.g. O, F or N.
  • The small H atom approaches close to other atoms - forms hydrogen bond.
  • Needs to be a lone pair of electrons on O, F or N atom to which the hydrogen can line up with.

Hydrogen bonds occur between molecules in liquid hydrogen flouride.

Each hydrogen atom needs a partial positive charge - bonded to a highly electronegative fluorine atom. 

Positively charged H atom lines up with a lone pair on a flourine atom in another HF molecule.

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Hydrogen Bonding in Water and Ice

A water molecule = two hydrogen atoms and one oxygen atom, covalently bonded together.

Oxygen has two bonding pairs and two pairs of lone electrons.

Each water molecule forms 2 times as many hydrogen bonds as a hydrogen fluoride molecule.

Hydrogen bonding causes some of water's unusual properties.

E.g. When water freezes, solid ice formed has a very open structure.

All the oxygen atoms are arranged tetrehedrally with two covalent bonds to hydrogen atoms and two hydrogen bonds to neighbouring water molecules.

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Effects of Hydrogen Bonding

Compounds w/ hydrogen bonding = higher boiling points than compounds with similar relative molecular masses that do not.

Ethanol = higher boiling point than propane - more energy required to overcome the hydrogen bonds between ethanol molecules than instantaneous dipole-induced dipole bonds between propane molecules.

Hydrogen bonding = solubiliity of ethanol in water.

Ethanol molecules form hydrogen bonds with water molecules - the liquids mix.

Hydrogen bonding also affects the viscocity of liquids.

E.g. glycerol - each molecule has 3 -OH groups which allows lots of hydrogen bondnig to occur.

In terms of strength:

Hydrogen bonds > permenant dipole-permenant dipole > Instantaneous dipole-induced dipole.

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Dissolving Polymers

Polymers such as poly(ethenol) dissolve in water because of structure - have the ability to form hydrogen bonds.

-OH groups on the polymer chain can hydrogen bond with water molecules = polymer is soluble.

Solubitiy can be changed by altering proportion of -OH groups in polymer.

Less -OH molecules = less soluble.

Very large amount of -OH molecules = undergoes a large amount of hydrogen bonding - takes so much energy to pull molecules apart so polymer becomes virtually insoluble.

Poly(ethenol) is useful for making soluble laundry bags used in hospitals, dissolving seed coatings and dissolving stitches used in surgery.

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Alcohols

Alcohols all contain -OH functional group and names ending in -ol.

There are three types of alcohol - primary, secondary and tertiary:

  • Primary = at the end of a chain (one R group attached to the carbon that -OH is attached to)
  • Secondary = in the middle of a chain (two R groups attached to the carbon)
  • Tertiary = attached to a carbon that has no H atoms attached to it (three R groups attached to the carbon).

Oxidation of alcohols

-OH group can be oxidised using acidified potassium dichromate(VI) - -OH group oxidised to a carbonyl group (C=O).

At the same time Cr2O7(2-)(aq) is reduced to Cr(3+)(aq), turning from orange to green.

During oxidation reaction 2 hydrogen atoms are removed - one from oxygen atom and one from carbon atom.

Reaction conditions for ox of alcs = heat under reflux w/ excess acidified potassium/sodium dichromate(VI) solution.

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Products of Oxidation

Product depends on type of alcohol used: Primary produces an aldehyde, which further oxidises into a carboxylic acid - colour of reaction mixture changes orange to green as dichromate(VI) ions are reduced.

Aldehyde (H-C=O) required in situ = can be distilled out of reaction mixture as its produced - prevents further oxidation. If carboxylic acid (OH-C=O) is needed then continuously heat under reflux w/ excess potassium dichromate(VI) solution.

Secondary alcohols produce ketones - no further oxidation occurs and colour of reaction mixture goes from orange to green.

Tertiary alcohols don't undergo oxidation with acidified potassium dichromate(VI) - they don't have a hydrogen atom on C which -OH is attached to - colour remains orange.

Carbonyl compounds = aldehydes and ketones and any group containing C=O (carbonyl bond).

Dehydration = alcohols losing a water molecule to produce an alkene - an example of an elimination reaction (small molecule removed from a larger molecule leaving an unsaturated molecule).

Typical reaction conditions = Al2O3 catalyst at 300C and 1atm / refluxing w/ concentrated sulphuric acid.

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Infrared Spectroscopy

Infrared (IR) spectroscopy = useful technique to determine structure of organic compounds.

- helps to identify different types of covalent bonds and so identify functional groups in a molecule.

A bond between any two atoms is like a vibrating spring - each bond has its own natural frequency of vibration.

The vibration depends on the types of atoms forming the bond and the type of bond (single, double, triple).

Molecule exposed to IR radiation - each bond absorbs energy at a particular frequency - causes it to vibrate more vigourously.

Different bonds absorb different frequencies of IR radiation.

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Typical IR Spectrum

IR spectra have the following features:

  • X-axis = wavenumber (cm-1) - scale starts at around 4000 and descends left to right to about 500 wavenumbers (wavenumer = number of waves that fit in 1cm).
  • Y-axis = percentage transmittance - baseline is at the top (100%) and absorption signals are downward troughs.
  • Absorptions described as 'strong', 'medium' and 'hydrogen bonded' (broad).
  • Part of IR sprectrum below 1500cm-1 = fingerprint region. Arenes = complex absorption patterns in fingerprint regions (Every compound has a distinctive fingerprint regions caused by the molecule's skeleton (C-C, C-O bonds etc)).

Most prominent absorption bands in an IR spectrum matched with bonds in a table - e.g. C=O bond is in the region of 1700cm-1 and is intense.

Functional groups that are involved in hydrogen bonding (-OH or -NH) usually give broad absorptions.

Precise position of an absorption signal depends on environment of the bond in the molecule - so the wavenumber has a region instead e.g. 1700-1725

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