Carbon • Organic chemistry is the study of the chemistry of carbon compounds containing hydrogen. • The compounds may also contain other elements, such as oxygen, nitrogen, a halogen, or sulphur. • Carbon is unique, in that it forms more compounds than are known of all the other elements in the periodic table put together. • Many millions of organic compounds have been identified or synthesized, and it is worth considering why carbon is so unique compared with all other elements.
1. Carbon can form multiple bonds to itself and with atoms of other elements. • Carbon can form • single bonds (C – C), • double bonds (C = C), • triple bonds (C Ξ C) to itself. • As well as forming single bonds it can form double and triple bonds with other elements such as oxygen in ethanoic acid, CH3COOH, and nitrogen in hydrogen cyanide, HCN.
2.The strength of the C – C single bond and the C – H bond. • The strength of the C – C bond means that it is thermally quite stable, particularly when compared with the single bond formed between two atoms of other elements in group 4 of the periodic table. • The single covalent bond that carbon forms with hydrogen is also very strong, 412 kJ mol-1, when compared with the silicon to hydrogen bond, 318kJ mol-1. • This also contributes to the thermal stability of carbon compounds.
3. Carbon can form chains and rings • The ability of carbon to form strong bonds to itself means that it is able to form long chains of carbon atoms. • In some polymers there can be well over 1000 carbon atoms in a single chain. • The chains can also link to themselves to form cyclic or ring compounds, such as cyclohexane, C6H12, and benzene, C6H6. • This ability to form chains and rings is called catenation. • A chain of carbon atoms without any branching, i.e, no methyl, ethyl groups are called straight chain molecules those with methyl, ethyl group attached are called branched chain molecules.
4. Carbon cannot expand its octet of valence electrons • When it is bonded, a carbon atom has a share in a full octet of electrons, and the outer shell cannot expand to include any more electrons, so it cannot form any more bonds. • Silicon, also has a full octet when it is bonded to four other atoms, but this octet can be expanded (due to d orbital's). This makes silicon compounds much less stable kinetically
Carbon cannot expand its octet of valence electrons • Compare, carbon tetrachloride, CCl4 and silicon tetrachloride SiCl4. • Carbon tetrachloride is completely immiscible in water and does not react. • Silicon tetrachloride reacts vigorously in water to produce HCl, because a non-bonding pair of electronson an oxygen atom in the water molecule is able to expand the octet on the silicon atom during the reaction.
Alkanes: a homologous series • A homologous series is one in which all the members have the same general formula. The neighbouring members of the series differ by -CH2, and they show similar chemical properties and a gradation in their physical properties. • The first four members of the homologous series of the alkanes are • Methane • Ethane • Propane • Butane • All four are gases under standard conditions of (298K, 1 atm) although their boiling points increase as the number of carbon atoms increases.
Representing molecules • There are many ways in which we can represent organic molecules; • Molecular formula • Structural formula • Displayed • Geometrical • Lewis (dot and cross) • How many structures can you draw for ethane?
Structures for the first four alkanes This shows the displayed formulae of the molecules and all their bonds but it gives the impression that the bonds angles are 90o to each other whereas we already know that they are, in fact, 109.5o and a tetrahedral shape. H H H H H C H C C H Ethane Methane H Butane H H H H H H H H H Propane H H H C C C H C C C C H H H H H H H
Structural isomers • C4H10 we could call butane because it has 4 carbons but would it be correct? • How many different ways are there to draw C4H10 remember that each carbon must have four bonds? • These are called structural isomers, they are molecules with the same molecular formula but different structural formulae. • Drawing displayed formulae for all molecules is tedious so we can write structured formulae or use the comprehensive IUPAC method of naming molecules to identify the differences.
Structured formulae • The condensed structural formulae for C4H10 can be; • CH3CH2CH2CH3 butane OR • CH3(CH2)2CH2 butane OR • CH3CH(CH3)CH3 methylpropane
Activity • Complete the practice questions on; Alkanes, Formulae and Isomers • Answers
Naming organic molecules • Before chemists understood the patterns in organic chemistry they named compounds randomly and many compounds are still known today by their common name. • But as more and more compounds were discovered and patterns emerged it was agreed by the IUPAC (International Union of Pure and Applied Chemistry) to name the compounds according to rules – it is called the IUPAC system for naming organic molecules.
IUPAC rules • Find the longest chain of carbon atoms, this will provide the stem of the name i.e. 1 carbon = meth, 2 carbons = eth etc • Identify any double or triple bonds. If all single then the ending will be ane, double bonds = ene, and triple bonds = yne • Look for other alkyl groups attached to the chain, i.e. CH3 = methyl, C2H5 = ethyl etc • Identify anyfunctional groups attached, -OH = alcohol etc • Starting from the end closest to the attached group or functional group, number the carbon that the group is attached to. • If there is more than one group attached name them in alphabetical order, functional groups first.
Example 1 H H C C H H H H H H H H C C C C H H H H
Example 1 H H C C H H H H H H H H C C C C H H H H There are five carbons in the longest chain, and all the carbon – carbon bonds are single so it is a pentane
Example 1 H H C C H H H H H H H H C C C C H H H H There is one methyl group attached
Example 1 H H C 5 H H H H H H H H 1 2 4 3 H H H H The methyl group is attached to carbon 2 so the molecule is called 2 methyl pentane
Example 2 H H C C H H H H H H H H H C C C C C H H H H H C H H
Example 2 H H C C H H H H H H H H H C C C C C H H H H The longest carbon chain is 5 so it has the stem pent- and all bonds are single so the ending is -ane H C H H
Example 2 H H C C H H H H H H H H H C C C C C H H H H H C H There are three methyl groups so we call it trimethyl H
Example 2 H H C C H H H H H H H H H 1 5 4 2 3 H H H H H C H There are 2 methyls on carbon 2 and one on carbon 4 H
Example 2 H H C C H H H H H H H H H 1 5 4 2 3 H H H H H C H This molecule is therefore called 2,2,4 trimethyl pentane H
Example 2 H H C C H H H H H H H H H C C C C C H H H H H C H It doesn’t matter how we derive the longest chain the name will always be the same. H
Example H H C C H H H H H H H H H C C C C C H H H H H C H H
General Properties of alkanes • Boiling points. • Plot the following boiling points and suggest a boiling temperature for pentane according to your graph. Describe the trend of your graph and give a reason for this trend in terms of bonding and structure.
Boiling points of the alkanes • The boiling points of alkanes are very low for their molar mass. • This is because the electronegativity values for carbon and hydrogen are very similar, and so the molecules are either completely non-polar because of their symmetrical shape or have very low polarity. • The boiling points increase as the molar mass of the alkane increases. • This is because the relatively weak van der Waals forces of attraction between the molecules increases with molar mass. • As you saw in the graph the actual increase in the boiling point becomes less each time an additional carbon atom is added to the chain • This is because the percentage change in molar mass decreases.
The actual increase in the boiling point becomes less each time an additional carbon atom is added to the chain. The boiling points increase as the molar mass of the alkane increases. The boiling points of alkanes are very low for their molar mass
Boiling point of structural isomers • Different structural isomers will have different boiling points. For example the three structural isomers of pentane have boiling points ranging between 9.5oC and 36.3oC; H H H H H 2-methylbutane b.p. 27.9oC H H c H c c c c c H H H H H H H H H H c c c c H H Pentane b.p. 36.3oC H H H H H c H H H H 2,2-dimethylpropane b.p.9.5oC H c c c H c The more spherical the isomer then the lower the boiling point as there will be less surface area to form intermolecular forces of attraction H H H H H
Stereoisomers • Structural isomers are compounds that possess the same molecular formula but have a different structural formula. • That is, the atoms are bonded in a different order: for example propan-1-ol and propan-2-ol • Stereoisomers have the same molecular formula and structural formula, but their atoms are arranged differently in space. • In stereoisomers each atom is bonded to the same atoms, but the way in which they are bonded is different. • There are two types of stereoisomerism • Geometrical • Optical
Geometrical • Geometrical isomerism occurs when bonds are unable to rotate freely. • This is known as restricted rotation. • This type of isomerism occurs in alkenes when the two atoms attached to the carbon atoms constituting the double bond are different. • Consider; but-1-ene and but-2-ene. • Compared with each other they are structural isomers, because the double bond is in a different place H H H H C C C C CH3 H CH3 C2H5
But-1-ene • If we could rotate the double bond in but-1-ene, then the outcome would be the same: the two hydrogen atoms on the end carbon atom simply exchange places, so there is only one compound of but-1-ene. H H C C H C2H5
But-2-ene • If we could rotate the double bond in but-2-ene, the outcome would be different. • In one case the two methyl groups are on the same side of the molecule, whereas in the other case they are on opposite sides, and clearly the distance between the end two carbon atoms is different. • When they are on the same side the compound is known as the cis- isomer • When they are on opposite sides (across the molecule) the compound is known as the trans- isomer. H C C H Cis-isomer Trans-isomer CH3 CH3
The cis- and trans- isomers exist separately, because the double bond cannot be rotated. • The carbon atoms on either side of the double bond are sp2 hybridised. • One of the bonds between the two carbon atoms is a sigma (σ) bond formed by the overlap of two sp2 hybrid orbitals, but the other bond is a pi (π) and formed by the sideways overlap of the p atomic orbitals on each carbon atom. • For the two p orbitals to overlap they must both be in the same plane; that is, they must both be pz orbitals or py orbitals. • Any attempt to rotate one of the carbon atoms relative to the other moves the p orbitals out of the same plane and thereby causes the bond to break • As this would require a considerable amount of energy the cis- and trans- isomers exist independently; they are not easily interconvertible, thanks to this high activation energy barrier.
Properties of cis- trans- isomers • The physical and chemical properties of cis- and trans- isomers can sometimes be similar, but often they are very different. • If there is a difference in melting points, for example, this may be due to the way in which they can pack together. • It is not easy to generalize about which isomer will have the higher melting point. • For example, cis-but-2-ene melts at -139˚C, which is lower than the melting point of trans-but-2-ene (-106˚C. • By contrast, cis-1,2-dichloroethene melts are 60.3˚C, which is higher than the melting point of trans-1,2-dichloroethene (47.5˚C)
Effect of Functional groups • Sometimes the differences can be even more marked, thanks to the nature of the functional groups attached to the double bond. • The melting points of cis-but-2-ene-1,4-dioic acid and trans-but-2-ene-1,4-dioic acid are very different. • In the trans- isomer there is a strong intermolecular hydrogen bonding between different molecules, because of the polarity of the carboxylic acid groups. • In the cis- isomer much of this hydrogen bonding occurs internally between the two acid groups. • This means the attraction between two different molecules is less, so that the melting point is much lower.
Differences in m.p O Trans- isomer Cis- isomer • The two geometric isomers are also different chemically. • In the cis- isomer the two carboxylic acid functional groups are close enough together to react, so that when the isomer is heated water can be expelled, and the cyclic acid anhydride is formed. • No such reaction is possible with the trans- isomer. δ- δ+ δ+ δ- O H H O H H Strong hydrogen bonding between molecules m.p. 286˚C Strong hydrogen bonding within molecules m.p. 130 -131˚C C C O O C O C H H C O O C C H C H
Cyclic compounds • Geometrical isomerism is also possible in cyclic compounds. • Although there are only single bonds between the carbon atoms, the rigid structure of the ring prevents free rotation. • Thus 1,2-dichloropropane, for example, can exist as cis- and trans- isomers H H C C H H Cl H H H C C C C H Cl Cl Cl
Optical Isomerism • Optical isomers occur when there are four different atoms or groups attached to a single carbon atom. • This is known as an asymmetric carbon atom. • Molecules containing an asymmetric carbon atom are said to be chiral molecules, although the word “chiral” is also sometimes used instead of “asymmetric” to just describe the carbon atom itself that contains the four different atoms of groups attached to it.
Activity: • Using the model kits create two optical isomers, you will need: • Two carbon atoms (these have four spokes) • 2 x white atoms (with just one spoke) • 2 x blue atoms (with just one spoke) • 2 x yellow atoms (with just one spoke) • 2 x green atoms (with just one spoke) • 8 straws
Enantiomers • You will see that, no matter how you try, it is impossible to superimpose one upon the other without breaking and remaking bonds. • The only way in which one can become the other is by reflection in a mirror: they are mirror images of each other. • These two mirror images are known as optical isomers or enantiomers. • Examples of simple molecules that contain an asymmetric carbon atom are butan-2-ol and 2-bromobutane. • The asymmetric carbon atom within a molecule is sometimes indicated using an asterisk
Three dimensional structures • Three dimensional structures can be represented in two dimensions by using a dotted line for bonds going behind the plane of the paper and wedge shaped lines for bonds coming out from the plane of the paper. • Draw the three dimensional structures of butan-2-ol and 2-bromobutane showing both enantiomers and indicating the chiral carbon using an asterisk The assymetric carbon atom within a molecule is sometimes indicated using an asterisk. W * Z C X Y
Physical properties of optical isomers • Optical isomers of a compound differ in only one respect in their physical properties: they rotate the plane of polarised light in opposite directions. • All their other physical properties, such as density and melting point, are identical. • The ability of enantiomers to rotate the plane of plane-polarized light can be shown in practice by using a polarimeter. • This consists of a light source, two polarising lenses, and a tube to hold the sample of the enantiomer located between the lenses.
If the analyser had to be rotated anticlockwise (to the left) the enantiomer is said to be laevorotatory, if it has to be rotated clockwise (to the right) it is dextrorotatory Polarimeter In order to see the light the analyser needs to be rotated to the same degree as the light has been rotated by the enantiomer. The enantiomer in the sample compartment causes the plane polarised light to rotate either clockwise or anti clockwise. When light passes through the first polarising lens it becomes plane-polarised; that is it vibrates in a single plane
Racemic Mixture • The two enantiomers rotate the plane of plane-polarised light by the same amount but in opposite directions. • One of the enantiomers is thus known as the d-form and the other as the l-form. • If both enantiomers are present in equal amounts the two rotations cancel each other out, and the mixture appears to be optically inactive. • Such a mixture is known as a racemic mixture.
Chemical properties of enantiomers • Not only are the physical properties of the two enantiomers of a chiral compound identical; their chemical properties are also identical – except when they interact with other optically active substances. • This is often the case in the body, where the different enantiomers can have completely different physiological, effects. • For example, one of the enantiomers of the amino acid asparagine H2N-CH(CH2CONH2)-COOH tastes bitter, whereas the other enantiomer tastes sweet. • Try drawing the two enantiomers
Amino acids • One important group of molecules that can show optical activity is the 2-amino acids. • Apart from the simplest, 2-aminoethanoic acid (glycine), in which the R group is a hydrogen atom, all the others can exist in both d- and l-forms. • Interestingly, in biological systems most amino acids occur as the l-form, whereas sugars, which can also show optical activity, tend to exist as the d-form. • Some people have speculated that this shows that all life originated from a single ancestor. • Others argue that it was simply a random event that occurred during the early stages of evolution
Solubility • The alkanes are non-polar. Or have extremely low polarity, so they do not dissolve in polar solvents such as water. • If they are liquids, then when mixed with water they simply form two immiscible layers. • However they can be good solvents in their own right, because they dissolve other non-polar compounds such as fats and oils