alkana. introducing alkanes and cycloalkanes alkanes formulae : c n h 2n+2 alkanes are the simplest...

AlkanaAlkanaAlkanaAlkana

INTRODUCING ALKANES AND CYCLOALKANESINTRODUCING ALKANES AND CYCLOALKANES

AlkanesFormulae : CnH2n+2

Alkanes are the simplest family of hydrocarbons - compounds containing carbon and hydrogen only. They only contain carbon-hydrogen bonds and carbon-carbon single bonds. The first six are: methane CH4, ethane C2H6,

propane C3H8, butane C4H10, pentane C5H12, hexane C6H14

IsomerismAll the alkanes with 4 or more carbon atoms in them show structural

isomerism. This means that there are two or more different structural formulae that you can draw for each molecular formula.

For example, C4H10 could be either of these two different molecules:These are

called respectively butane and 2-methylpropane.


CycloalkanesCycloalkanes again only contain carbon-hydrogen

bonds and carbon-carbon single bonds, but this time the carbon atoms are joined up in a ring. The smallest cycloalkane is cyclopropane. If you count the carbons and hydrogens, you will see that they no longer fit the general formula CnH2n+2. By joining the carbon atoms in

a ring, you have had to lose two hydrogen atoms. You are unlikely to ever need it, but the general formula for a cycloalkane is CnH2n


Don't imagine that these are all flat molecules. All the cycloalkanes from cyclopentane upwards exist as "puckered rings". Cyclohexane, for example, has a ring structure which looks like this: This is known as the "chair" form of cyclohexane - from its shape which vaguely resembles a chair.


Physical PropertiesThe boiling points shown are all for the "straight chain" isomers

where there are more than one. Notice that the first four alkanes are gases at room temperature. Solids don't start to appear until about C17H36. You can't be more precise than that

because each isomer has a different melting and boiling point. By the time you get 17 carbons into an alkane, there are unbelievable numbers of isomers! Cycloalkanes have boiling points which are about 10 - 20 K higher than the corresponding straight chain alkane.


ExplanationsThere isn't much electronegativity difference between

carbon and hydrogen, so there is hardly any bond polarity. The molecules themselves also have very little polarity.

A totally symmetrical molecule like methane is completely non-polar. This means that the only attractions between one molecule and its neighbours will be Van der Waals dispersion forces. These will be very small for a molecule like methane, but will increase as the molecules get bigger. That's why the boiling points of the alkanes increase with molecular size.


Where you have isomers, the more branched the chain, the lower the boiling point tends to be. Van der Waals dispersion forces are smaller for shorter molecules, and only operate over very short distances between one molecule and its neighbours. It is more difficult for short fat molecules (with lots of branching) to lie as close together as long thin ones.

For example, the boiling points of the three isomers of C5H12 are: boiling point (K) of pentane, 309.22;

methylbutane, 301.02; 2-dimethylpropane, 282.6. The slightly higher boiling points for the cycloalkanes are presumably because the molecules can get closer together because the ring structure makes them tidier and less "wriggly"!

SolubilitySolubility

SolubilityThe factsWhat follows applies equally to alkanes and

cycloalkanes. Alkanes are virtually insoluble in water, but dissolve in organic solvents. The liquid alkanes are good solvents for many other covalent compounds.


ExplanationsSolubility in waterWhen a molecular substance dissolves in water,

you have to• break the intermolecular forces within the

substance. In the case of the alkanes, these are Van der Waals dispersion forces.

• break the intermolecular forces in the water so that the substance can fit between the water molecules. In water the main intermolecular attractions are hydrogen bonds.


Breaking either of these attractions costs energy, although the amount of energy to break the Van der Waals dispersion forces in something like methane is pretty negligible. That isn't true of the hydrogen bonds in water, though.

As something of a simplification, a substance will dissolve if there is enough energy released when new bonds are made between the substance and the water to make up for what is used in breaking the original attractions.

The only new attractions between the alkane and water molecules are Van der Waals. These don't release anything like enough energy to compensate for what you need to break the hydrogen bonds in water. The alkane doesn't dissolve.


Solubility in organic solventsIn most organic solvents, the main forces of attraction

between the solvent molecules are Van der Waals - either dispersion forces or dipole-dipole attractions.

That means that when an alkane dissolves in an organic solvent, you are breaking Van der Waals forces and replacing them by new Van der Waals forces. The two processes more or less cancel each other out energetically - so there isn't any barrier to solubility.

Chemical ReactivityChemical Reactivity

AlkanesAlkanes contain strong carbon-carbon single bonds and

strong carbon-hydrogen bonds. The carbon-hydrogen bonds are only very slightly polar and so there aren't any bits of the molecules which carry any significant amount of positive or negative charge which other things might be attracted to. The net effect is that alkanes have a fairly restricted set of reactions.


The net effect is that alkanes have a fairly restricted set of reactions.

You can• burn them - destroying the whole molecule;• react them with some of the halogens, breaking

carbon-hydrogen bonds;• crack them, breaking carbon-carbon bonds.These reactions are all covered on separate pages

if you go to the alkanes menu (see below).


CycloalkanesCycloalkanes are very similar to the alkanes in reactivity,

except for the very small ones - especially cyclopropane. Cyclopropane is much more reactive than you would expect.

The reason has to do with the bond angles in the ring. Normally, when carbon forms four single bonds, the bond angles are about 109.5°. In cyclopropane, they are 60°. With the electron pairs this close together, there is a lot of repulsion between the bonding pairs joining the carbon atoms. That makes the bonds easier to break.

THE NAMES OF ORGANIC COMPOUNDSTHE NAMES OF ORGANIC COMPOUNDS

Cracking the codeA modern organic name is simply a code. Each part of the

name gives you some useful information about the compound. For example, to understand the name 2-methylpropan-1-ol you need to take the name to pieces. The prop in the middle tells you how many carbon atoms there are in the longest chain (in this case, 3). The an which follows the "prop" tells you that there aren't any carbon-carbon double bonds. The other two parts of the name tell you about interesting things which are happening on the first and second carbon atom in the chain. Any name you are likely to come across can be broken up in this same way.

THE NAMES OF ORGANIC COMPOUNDSTHE NAMES OF ORGANIC COMPOUNDS

Counting the carbon atomsYou will need to remember the codes for the

number of carbon atoms in a chain up to 6 carbons. There is no easy way around this - you have got to learn them. If you don't do this properly, you won't be able to name anything!

codeno of carbons; meth, 1;eth, 2; prop, 3; but, 4; pent, 5; hex, 6

THE NAMES OF AROMATIC COMPOUNDSTHE NAMES OF AROMATIC COMPOUNDS

BackgroundThe benzene ringAll aromatic compounds are based on benzene, C6H6, which has a

ring of six carbon atoms and has the symbol:

Each corner of the hexagon has a carbon atom with a hydrogen

attached. The phenyl groupRemember that you get a methyl group, CH3, by removing a

hydrogen from methane, CH4. You get a phenyl group, C6H5, by

removing a hydrogen from a benzene ring, C6H6. Like a methyl

or an ethyl group, a phenyl group is always attached to something else.

Aromatic compounds with only one Aromatic compounds with only one group attached to the benzene ringgroup attached to the benzene ring

Cases where the name is based on benzenechlorobenzeneThis is a simple example of a halogen attached to the benzene ring. The name is self-obvious. The

simplified formula for this is C6H5Cl. You could therefore (although you never do!) call it

phenyl chloride. Whenever you draw a benzene ring with one other thing attached to it, you are in fact drawing a phenyl group. In order to attach something else, you have to remove one of the existing hydrogen atoms, and so automatically make a phenyl group.

nitrobenzeneThe nitro group, NO2, is attached to a benzene ring. The simplified formula for this

is C6H5NO2.

methylbenzeneAnother obvious name - the benzene ring has a methyl group attached. Other alkyl

side-chains would be named similarly - for example, ethylbenzene. The old name for methylbenzene is toluene, and you may still meet that.The simplified formula for this is C6H5CH3

Aromatic compounds with only one Aromatic compounds with only one group attached to the benzene ringgroup attached to the benzene ring

Cases where the name is based on benzenechlorobenzeneThis is a simple example of a halogen attached to the

benzene ring. The name is self-obvious. The simplified formula for this is C6H5Cl. You could therefore (although

you never do!) call it phenyl chloride. Whenever you draw a benzene ring with one other thing attached to it, you are in fact drawing a phenyl group. In order to attach something else, you have to remove one of the existing hydrogen atoms, and so automatically make a phenyl group.

the name is based on the name is based on benzenebenzene

If more than one of the hydrogens had been replaced by chlorine, the names would be (dichloromethyl)benzene or (trichloromethyl)benzene. Again, notice the importance of the brackets in showing that the chlorines are part of the side group and not directly attached to the ring.

benzoic acid (benzenecarboxylic acid)Benzoic acid is the older name, but is still in common use - it's a lot

easier to say and write than the modern alternative! Whatever you call it, it has a carboxylic acid group, -COOH, attached to the benzene ring.

the name is based on the name is based on phenylphenyl

phenylaminePhenylamine is a primary amine and contains the -NH2

group attached to a benzene ring. The old name for phenylamine is aniline, and you could also reasonably call it aminobenzene.

phenyletheneThis is an ethene molecule with a phenyl group attached.

Ethene is a two carbon chain with a carbon-carbon double bond. Phenylethene is therefore: The old name for phenylethene is styrene - the monomer from which polystyrene is made.

the name is based on the name is based on phenylphenyl

phenylethanoneThis is a slightly awkward name - take it to pieces. It consists

of a two carbon chain with no carbon-carbon double bond. The one ending shows that it is a ketone, and so has a C=O group somewhere in the middle. Attached to the carbon chain is a phenyl group. Putting that together gives:

phenyl ethanoateThis is an ester based on ethanoic acid. The hydrogen atom

in the -COOH group has been replaced by a phenyl group.phenolPhenol has an -OH group attached to a benzene ring and so

has a formula C6H5OH.

Aromatic compounds with more Aromatic compounds with more than one group attached to the than one group attached to the benzene ringbenzene ring

Substituting chlorine atoms on the ringLook at these compounds:

All of these are based on methylbenzene and so the methyl group is given the number 1 position on the ring.

Why is it 2-chloromethylbenzene rather than 6-chloromethylbenzene? The ring is numbered clockwise in this case because that produces a 2- in the name rather than a 6-. 2 is smaller than 6.


2-hydroxybenzoic acidThis might also be called 2-hydroxybenzenecarboxylic acid.

There is a -COOH group attached to the ring and, because the name is based on benzoic acid, that group is assigned the number 1 position. Next door to it in the 2 position is a hydroxy group, -OH.

benzene-1,4-dicarboxylic acidThe di shows that there are two carboxylic acid groups, -

COOH, one of them in the 1 position and the other opposite it in the 4 position.


2,4,6-trichlorophenolThis is based on phenol - with an -OH group attached in the number 1

position on the ring. There are 3 chlorine atoms substituted onto the ring in the 2, 4 and 6 positions. 2,4,6-trichlorophenol is the familiar antiseptic TCP.

methyl 3-nitrobenzoateThe structure of the name shows that it is an ester. You can tell that

from the oate ending, and the methyl group floating separately from the rest of the name at the beginning.The ester is based on the acid, 3-nitrobenzoic acid - so start with that.There will be a benzene ring with a -COOH group in the number 1 position and a nitro group, NO2, in the 3 position. The -COOH group is modified to make an

ester by replacing the hydrogen of the -COOH group by a methyl group.

alkana. introducing alkanes and cycloalkanes alkanes formulae : c n h 2n+2 alkanes are the simplest...

Documents

introducing alkanes

cycloalkanes alkanes

isomers of c

carbon atoms

carboncarbon single

carbonhydrogen bonds

general formula c n

butane c