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5.6 12.2 Structure and properties of polymers Alkenes 5.3 Bonds between molecules - temporary and permanent dipoles (revisited) 5.4 Bonds between molecules - hydrogen bonding 13.2 Alcohols and ethers 3.4 E/Z isomerism 6.4 Infrared spectroscopy Chemical Ideas 5.6 The structure and properties of polymers (Part 1) A polymer molecule is a long molecule made up from lots of small molecules called monomers. If all the monomers are the same an A-A polymer forms: A + A + A + A A-A-A-A Poly(ethene) and poly(chloroethene) or PVC are examples of A-A polymers. If two different monomers are used an A-B polymer may be formed in which A and B monomers alternate along the chain: A + B + A + B A-B-A-B Polyamides (nylons) and polyesters are examples of A-B polymers. Addition polymerisation Many polymers are formed in a reaction known as addition polymerisation. The monomers usually contain C=C double bonds e.g. in alkenes. In the chain, the same basic unit is repeated over and over again. What decides the properties of a polymer? Strength and flexibility of polymers depend on forces of attraction between the polymer chains; Generally, the stronger these inter-molecular forces are , the stronger the polymer is; and the less flexible the molecule is; For a polymer to be flexible, the chains must be able to slide past each other easily. The forces of attraction depend on the structure of the polymer – especially: chain length – longer chains = stronger polymers; side groups – polar side chains = stronger attraction between chains making the polymer stronger; branching – highly branched chains cannot pack closely together = weaker attraction = polymer is weak; stereoregularity – polymer chains pack together more closely if side chains are orientated in a particular way. These are called isotactic polymers. chain flexibility cross linking – this makes the polymer harder and more difficult to melt; Thermoplastics and thermosets Crystalline polymers Amorphous regions = the polymer chains are arranged in a random order Crystalline regions = the polymer chains are packed in a regular arrangement It is important to know the degree of crystallinity in a polymer as the higher it is the stronger and less flexible the polymer. Cold drawing When a polymer is stretched (cold-drawn) a neck forms. In the neck, the polymer chains line up to form a more crystalline region. Cold drawing leads to a large increase in the polymer’s strength. Chemical Ideas 12.2 Alkenes Alkenes are distinguished from other hydrocarbons by the presence of the C=C double bond. The double bond implies that they are unsaturated hydrocarbons. As with the alkanes, the boiling points of alkenes increase as the number of carbon atom increases. Ethene, propene and butene are gases. After that they are liquids and eventually solids. Ethene is a planar molecule with a bond angle of approximately 120° but propene is not a planar molecule. In propene: the part of the molecule around the double bond is planar with a bond angle of approximately 120° the methyl group –CH3 has three hydrogen atoms in a tetrahedral arrangement with a bond angle of approximately 109° There is a convention for representing the three-dimensional shape of ethene, propene and other organic chemicals on paper: solid line represents a covalent bond in the plane of the paper a dashed line represents a covalent bond going into the plane of the paper a wedge represents a covalent bond coming out of the plane of the paper H H C C H C H H H Naming alkenes The first part of the name of an alkene depends upon the number of carbon atoms in the longest chain. The names of alkenes end in –ene. For example, ethene has two carbon atoms, butene has four and hexene has six. For alkenes with more than three carbon atoms, positional isomers are possible. The position of the double bond is included in the name. The carbon atoms are numbered starting at 1, in such a way that the lowest possible number can be used. For example: CH3CH2CH=CH2 is but-1-ene and not but-3-ene or but-4-ene CH3CH=CHCH3 is but-2-ene and not but-3-ene If a branch is present, the alkene is named after the longest chain containing the double bond. The name of the branch itself is added to the start of the name. So, methylpropene is propene with a CH3 group attached. Dienes have two double bonds in their structure. Cyclic alkenes are named cyclo-, followed by the prefix determined by the number of carbon atoms in the ring and finally the –ene part. So a six-carbon ring with a double bond is called cyclohexene. Chemical reactions of ethene The four electrons in the double bond of ethene give the region between the two carbon atoms a higher than normal density of negative charge. Positive ions or molecules with a partial positive charge will be attracted to this negatively charged region. These are described as electrophiles – they are attracted to this negatively charged region and accept a pair of electrons from the C=C double bond to form a covalent bond. Electrophilic addition reactions When we bubble ethene gas through bromine, the red-brown bromine becomes decolourised – this is a good general test for unsaturation in an organic compound. The bromine molecule becomes polarised as it approaches the alkene – the electrons in the bromine are repelled by the alkene electrons and are pushed back along the molecule. The bromine atom nearest the alkene becomes slightly positively charged and the bromine atom furthest from the alkene becomes slightly negatively charged. The positively charged bromine atom now behaves as an electrophile and reacts with the alkene double bond. Notice that one of the carbon atoms now has a share in only six outer electrons and becomes positively charged – this is called a carbocation. H H H C = C H H H + C C H + .. : Br : .. - H Br a carbocation and a bromide ion + Br δ Br δ Carbocations react very rapidly with anything that has electrons to share such as the bromide ion. A pair of electrons moves from the Br- ion to the positively charged carbon to form a new C-Br covalent bond. H H C H + C H H Br .. : Br : .. H H C C Br Br H - This is an addition reaction and since the initial attack is by an electrophile the process is called an electrophilic addition. Reaction with hydrogen bromide Ethene reacts readily at room temperature with a solution of HBr in a polar solvent. It is another example of electrophilic addition. Reaction with water In the presence of a catalyst (phosphoric acid adsorbed onto solid silica) and high temperature and pressure, ethene and water (steam) undergo an addition reaction. The process is used for the industrial manufacture of ethanol. The addition of water to an alkene is an example of a hydration reaction. Reaction with hydrogen This is another example of an addition reaction. A catalyst is required (usually platinum) to break the strong H-H bond and form H atoms. Addition of hydrogen to an alkene is known as hydrogenation and is used in the manufacture of margarine in which plant an animal oils (unsaturated) are hardened as they become saturated molecules. Chemical Ideas 5.3 Bond between molecules: temporary and permanent dipoles (revisited) The electrons in a molecule are moving constantly. At any one time the electron cloud may be distributed unequally which causes an instantaneous dipole on the molecule. This induces a dipole on a neighbouring molecule. Permanent dipoles Permanent dipoles arise where an atom is covalently bonded to a more electronegative atom. Electrostatic forces The partial charges are shown by the symbols δ- and δ+. There is an electrostatic force of attraction between the δ- charge on one molecule and the δ+ charge on the neighbouring molecule. Instantaneous dipole-induced dipole attractions These attractions are the weakest type of intermolecular force. All simple covalent compounds have them. The strength of instantaneous dipole-induced dipole attractions increases: as the size of the molecule increases. This is because the size of the charged cloud increases so it is more easily polarised. as the amount of contact between molecules increases. In alkanes, there is more contact between straight chain molecules than between branched chain molecules. Molecules with branches have weaker instantaneous dipoleinduced dipole attractions and lower boiling points than molecules without branches. Permanent dipole-permanent dipole attractions These are stronger than instantaneous dipole-induced dipole attractions. Compounds that also have these intermolecular forces have higher boiling points than those who do not. H δ+ Cl δ- H δ+ Cl δ- Chemical Ideas 5.4 Bonds between molecules: hydrogen bonding Hydrogen bonds are intermolecular forces of attraction. Water is a common compound with hydrogen bonds. They are permanent dipole-permanent dipole attractions that occur in a particular set of circumstances. For hydrogen bonding to occur in a molecule, there needs to be: a hydrogen atom covalently bonded to a small, electronegative atom (nitrogen, oxygen or fluorine) and at least one lone pair of electrons on the electronegative atom; Although chlorine is as electronegative as oxygen, it is too large to form a hydrogen bond. Just having hydrogen atoms in a molecule is not sufficient for hydrogen bonding: methane, CH4, does not have hydrogen bonding. Hydrogen bonds are the strongest type of intermolecular force of attraction. Molecules with hydrogen bonding have higher boiling points than would be expected from their relative formula masses alone. Methane and ammonia have similar relative formula masses and so similar instantaneous dipole-induced dipole attractions. However, ammonia has a much higher boiling point because it also has hydrogen bonds. Chemical Ideas 13.2 Alcohols and ethers Alcohols Alcohols contain the hydroxyl group –OH. The first part of their name depends upon the number of carbon atoms in the longest chain. The names of alcohols end in –ol. For example, ethanol has two carbon atoms, butanol has four and hexanol has six. For alcohols with three or more carbon atoms, position isomers are possible. The carbon atoms are numbered starting at 1, in such a way that the lowest total number will be obtained. The position of the hydroxyl group is included in the name. For example: CH3CH2CH2OH is propan-1-ol and not propan-3-ol There are primary, secondary and tertiary alcohols: in primary alcohols, the carbon atom carrying the hydroxyl group is bonded directly to one other carbon atom; in secondary alcohols, the carbon atom carrying the hydroxyl group is bonded directly to two other carbon atoms; and in tertiary alcohols, the carbon atom carrying the hydroxyl group is bonded directly to three other carbon atoms; Alcohols with more than one hydroxyl group are possible. These are called polyhydric alcohols: diols have two hydroxyl groups triols have three hydroxyl groups Physical properties of alcohols Like water molecules, alcohol molecules are polar because of the polarised O-H bond. In both water and alcohols, there is a special sort of strong attractive force between the molecules due to hydrogen bonds. Hydrogen bonding between alcohol and water molecules explains why the two liquids mix together. As the hydrocarbon chain in alcohols becomes longer and the molecules become larger, the influence of the –OH group on the properties of the molecule becomes less important. So the properties of the higher alcohols get more and more like those of the corresponding alkane. Reactions of alcohols Oxidation The –OH group can be oxidised by strong oxidising agents such as acidified potassium dichromate(VI). The orange dichromate(VI) ion, Cr2O72-(aq) is reduced to green Cr3+(aq). In this reaction two atoms of hydrogen are removed – one from the oxygen atom and one from the carbon atom so oxidation will not take place unless there is a hydrogen atom on the carbon atom to which the –OH is attached. The product is a carbonyl compound – an aldehyde or a ketone. The type of product depends on the type of alcohol you start with. Primary alcohols such as ethanol are oxidised to aldehydes but the aldehyde is then oxidised itself to a carboxylic acid. If the aldehyde is required, the aldehyde is distilled out from the reaction mixture before it is oxidised further. Secondary alcohols such as propan-2-ol are oxidised to ketones. Tertiary alcohols such as 2-methylpropan-2-ol are difficult to oxidise because they do not have a hydrogen atom on the carbon atom to which the –OH group is attached. Dehydration of alcohols Many alcohols can lose a molecule of water to form an alkene e.g. propene is formed when vapour of propan-1-ol is passed over a hot catalyst of alumina at 300°C. The reaction is described as dehydration since it involves the removal of a water molecule from the molecule of the reactant. Alcohols can also be dehydrated by heating with concentrated sulphuric acid. Dehydration is an example of an elimination reaction. Ethers Ethers are derived from alkanes by substituting an alkoxy group (-OR) for an H atom. General formula of ethers R-OR’. The longer hydrocarbon chain is chosen as the parent alkane for naming. Ethers can be thought of as being derived from water by replacing both the H atoms with alkyl groups. Ether molecules are only slightly polar and the attractive force between the molecules are relatively weak. There are no H atoms attached to the oxygen to form hydrogen bonds between ether molecules. The boiling point of an ether is similar to that of the alkane with corresponding relative molecular mass. Like alkanes, the lower ethers are very volatile and dangerously flammable. Ethers are only slightly soluble in water but mix well with other non-polar molecules such as alkanes. Chemical Ideas 3.5 E/Z isomerism Isomers are molecules with the same molecular formula but different arrangements of their atoms. Alkenes can show a type of stereoisomerism called geometric isomerism. This is because the double bond resists rotation. For an alkene to show geometric isomerism, the carbon atoms attached by the double bond must both have different substitute groups attached to them. If one or more has two identical groups attached twice, the alkene does not show geometric isomerism. If the substitute groups are on the same side of the double bond, the isomer is called a cis isomer. If the groups are on the opposite sides of the double bond, the isomer is called a trans isomer. Geometric isomers have different physical properties such as melting point and density. H H H C H C H H C C H H cis-but-2-ene H H C H H C C H C H H trans-but-2-ene H Chemical Ideas 6.4 Infrared spectroscopy The energy possessed by molecules is quantised – molecules can only have a small number of definite energy values rather than any energy value. Analysis of the energy (or frequency of radiation) needed to produce a change from one energy level to another is the basis of most forms of molecular spectroscopy. In infrared spectroscopy substances are exposed to radiation in the frequency range 1014-1013Hz i.e. wavelengths 2.5-15μm. This makes vibrational energy changes occur in the molecules which absorb infrared radiation of specific frequencies. The important point to remember about infrared spectroscopy is that you do not try to explain the whole spectrum; you look for one or two signals which are characteristic of particular bonds. Infrared radiation from a heated filament is split into two parallel beams, one of which passes through the sample, the other through a reference chamber. This ensures that unwanted absorptions from water and carbon dioxide in the air or from a solvent are cancelled out. The beams are then directed by mirrors so that they follow parallel paths. The beams are analysed by passing them through a prism of sodium chloride which is transparent to infrared radiation or through a diffraction grating. Light of only one particular frequency will now be focused onto the detector. The spectrum is produced by rotating the prism so that the detector scans the frequencies and records their intensities. When the sample is not absorbing there will be no difference between the two beams reaching the detector so no signal is recorded. When a vibration is being excited, the sample absorbs radiation; the sample beam intensity will be reduced and a signal generated. A newer method called Fourier transform infrared (FTIR) spectroscopy uses a single beam of infrared radiation and the phenomenon of wave interference to produce an infrared spectrum. FTIR is cheaper and faster for collecting infrared data. Interpreting the spectra In general we can match a particular bond to a particular absorption region. The precise position of an absorption depends on the environment of the bond in the molecule so we can only quote wavenumber regions in which we can expect absorptions to arise. Below 1500cm-1 the spectrum can be quite complex and it is more difficult to assign absorptions to particular bonds. This region is characteristic of a particular molecule and is often called the fingerprint region. It is useful for identification purposes e.g. to compare two spectra to find out if they are spectra of the same compound. It is only rarely used to identify functional groups. This spectrum shows a strong absorption at 2970cm-1 characteristic of C-H stretching in aliphatic compounds. There is no indication of any functional groups. The spectrum of benzoic acid shows a sharp absorption at 3580cm-1 characteristic of an O-H bond (not hydrogen bonded). The strong absorption at 1760cm-1 shows the presence of a C=O group. The position of the C-H absorption suggests it is an aromatic compound.