Alkenes

• The alkanes are said to be saturated hydrocarbons because they contain only single carbon-carbon bonds,those with multiple bonds are called unsaturated hydrocarbons.

• The simplest compounds of this type are the alkenes, which contain a carbon–carbon double bond.

• The alkynes, containing C≡C triple bonds, are also unsaturated hydrocarbons.

• The general formula for the alkene homologous series is CnH2n

• This is also the general formula of the cycloalkanes. • The structural formulae and names of the simplest

isomers of the first five members of the series are given in Figure 1019.

Alkenes• Though double bonds are stronger than single bonds,

they are not twice as strong (C=C 612 kJ mol–1, C—C 348 kJ mol–1).

• This means that it is energetically favourable for a double bond to be converted into two single bonds.

• Theactivation energy for these reactions is also relatively low,owing to the high electron density in the double bond.

• This means that alkenes are considerably more reactivethan alkanes and are an important starting point in the synthesis of other organic compounds.

• As a result alkenes,usually formed by the cracking of fractions of petroleum,are very important intermediates in the economically important petrochemicals industry.

Alkenes are named according to standard IUPAC rules

• select the longest chain of C atoms containing the double bond; • place the ending ENE on the basic name• number the chain starting from the end nearer the double bond• use a number to indicate the lower number carbon of the C=C• as in alkanes, prefix with substituents• side chain positions are based on the number allocated to the first C of the C=C• if geometrical isomerism exists, prefix with cis or trans

e.g. CH3 - CH = CH - CH2 - CH(CH3) - CH3 is called 5-methylhex-2-ene

NAMING ALKENESNAMING ALKENES

STRUCTURAL ISOMERISM IN ALKENESSTRUCTURAL ISOMERISM IN ALKENES

Different structures are possible due to...

Different positions for the double bond

pent-1-ene pent-2-ene

Branching

3-methybut-1-ene

GEOMETRICAL ISOMERISM IN ALKENESGEOMETRICAL ISOMERISM IN ALKENES

• an example of stereoisomersim• found in some, but not all, alkenes• occurs due to the RESTRICTED ROTATION OF C=C bonds• get two forms....

CISGroups/atoms are on theSAME SIDE of the double

bond

TRANSGroups/atoms are on

OPPOSITE SIDES across the double bond

Isomers - have different physical properties - e.g. boiling points, density- have similar chemical properties - in most cases

GEOMETRICAL ISOMERISMGEOMETRICAL ISOMERISM

RESTRICTED ROTATION OF C=C BONDS

Single covalent bonds can easily rotate. What appears to be a different structure is not. It looks like it but, due to the way structures are written out, they are the same.

ALL THESE STRUCTURES ARE THE SAME BECAUSE C-C BONDS HAVE ‘FREE’ ROTATION

GEOMETRICAL ISOMERISMGEOMETRICAL ISOMERISM

RESTRICTED ROTATION OF C=C BONDS

C=C bonds have restricted rotation so the groups on either end of the bond are ‘frozen’ in one position; it isn’t easy to flip between the two.

This produces two possibilities. The two structures cannot interchange easily so the atoms in the two molecules occupy

different positions in space.

GEOMETRICAL ISOMERISMGEOMETRICAL ISOMERISM

How to tell if it exists

Two different atoms/groups attached

Two different atoms/groups attached

Two similar atoms/groups attached

Two similar atoms/groups attached

Two similar atoms/groups attached

Two different atoms/groups attached

Two different atoms/groups attached

Two different atoms/groups attached

GEOMETRICAL ISOMERISM

GEOMETRICAL ISOMERISM

Once you get two similar atoms/groups attached to

one end of a C=C, you cannot have geometrical

isomerism

GEOMETRICAL ISOMERISMGEOMETRICAL ISOMERISM

Isomerism in butene

There are 3 structural isomers of C4H8 that are alkenes*. Of these ONLY ONE exhibits geometrical isomerism.

BUT-1-ENE 2-METHYLPROPENEtrans BUT-2-ENEcis BUT-2-ENE

* YOU CAN GET ALKANES WITH FORMULA C4H8 IF THE CARBON ATOMS ARE IN A RING

Boiling point trends are similar to those shown in alkanesincreases as they get more carbon atoms in their formula

more atoms = greater intermolecular Van der Waals’ forces greater intermolecular force = more energy to separate molecules greater energy required = higher boiling point

the lower members are gases at room temperature and pressurecyclohexene C6H10 is a liquid

for isomers, greater branching = lower boiling point

C2H4 (- 104 °C) C3H6 (- 48°C) ....... C6H10 (83°C)

Melting point general increase with molecular massthe trend is not as regular as that for boiling point.

Solubilityalkenes are non-polar so are immiscible (don’t mix with) with watermiscible with most organic solvents.

PHYSICAL PROPERTIES OF ALKENESPHYSICAL PROPERTIES OF ALKENES

Addition Reactions of Alkenes• The most common chemical transformation of a

carbon-carbon double bond is the addition reaction.• A large number of reagents, both inorganic and

organic, have been found to add to this functional group.

• A majority of these reactions are exothermic, due to the fact that the C-C pi-bond is relatively weak ( 63 kcal/mole) relative to the sigma-bonds formed to the atoms or groups of the reagent.

• Remember, the bond energies of a molecule are the energies required to break (homolytically) all the covalent bonds in the molecule.

• Consequently, if the bond energies of the product molecules are greater than the bond energies of the reactants, the reaction will be exothermic.

• The following calculations for the addition of H-Br are typical. Note that by convention exothermic reactions have a negative heat of reaction.

Addition Reactions of Alkenes

Reactions of Alkenes

• A reaction in which the double bond of an alkene is converted to a single bond and two new bonds are formed to the species it reacts with is known as an addition reaction and they are typical of alkenes and alkynes.

• A number of important addition reactions are illustrated in the next slides named as:

• Halogenation• Halogen acid addition• Catalytic Hydrogenation• Addition of water

Addition of Strong Brønsted Acids • Strong Brønsted acids such as HCl, HBr, HI & H2SO4, rapidly

add to the C=C functional group of alkenes to give products in which new covalent bonds are formed to hydrogen and to the conjugate base of the acid.

• Animation below shows the addition products expected on reacting each of the above acids with cyclohexene.

                                                                                                        

          

Addition of Strong Brønsted Acids

• Weak Brønsted acids such as water (pKa = 15.7) and acetic acid (pKa = 4.75) do not normally add to alkenes.

• However, the addition of a strong acid serves to catalyze the addition of water, and in this way alcohols may be prepared from alkenes.

• For example, if sulfuric acid is dissolved in water it is completely ionized to the hydronium ion, H3O+, and this strongly acidic (pKa = -1.74) species effects hydration of ethene and other alkenes.

 

CH2=CH2   +   H3O+   ——>   HCH2–CH2OH   +   H+

Addition of water• With water (in the form of superheated steam), the addition

reaction is reversible. • At a temperature of ~ 300°C and a high pressure (~7 atm)

the equilibrium shown is driven to the right (Le Chatelier’s principle) and this provides the basis for the industrial manufacture of ethanol.

• Ethanol is used in large quantities by industry both as a solvent and as an intermediate in the manufacture of other compounds, hence this is a very commercially important process.

• At atmospheric pressure the equilibrium lies to the left and alkenes are formed by the dehydration of alcohols.

• The reaction in both directions is catalysed by either acids (usually H2SO4 or H3PO4) or heated aluminium oxide, Al2O3.

Manufacturing ethanol

• Ethanol is manufactured by reacting ethene with steam. The reaction is reversible.

• • Only 5% of the ethene is converted into ethanol at each

pass through the reactor. By removing the ethanol from the equilibrium mixture and recycling the ethene, it is possible to achieve an overall 95% conversion.

Manufacturing ethanolA flow scheme for the reaction looks like this:

When the gases from the reactor are cooled, then excess steam will condense as well as the ethanol. The ethanol will have to be separated from the water by fractional distillation.

Regioselectivity and the Markovnikov Rule

• Only one product is possible from the addition of these strong acids to symmetrical alkenes such as ethene and cyclohexene.

• However, if the double bond carbon atoms are not structurally equivalent, as in molecules of 1-butene, 2-methyl-2-butene and 1-methylcyclohexene, the reagent conceivably may add in two different ways.

• This is shown for 2-methyl-2-butene in the following equation.

Regioselectivity and the Markovnikov Rule

• When addition reactions to such unsymmetrical alkenes are carried out, we find that one of the two possible constitutionally isomeric products is formed preferentially.

• Selectivity of this sort is termed regioselectivity. In the above example, 2-chloro-2-methylbutane is nearly the exclusive product.

• Similarly, 1-butene forms 2-bromobutane as the predominant product on treatment with HBr.

The Markovnikov Rule

• After studying many addition reactions of this kind, the Russian chemist Vladimir Markovnikov noticed a trend in the structure of the favored addition product.

• He formulated this trend as an empirical rule we now call The Markovnikov Rule:  When a Brønsted acid, HX, adds to an unsymmetrically substituted double bond, the acidic hydrogen of the acid bonds to that carbon of the double bond that has the greater number of hydrogen atoms already attached to it.

Bromine Water Test

• The usual test for the presence of a carbon–carbon doubleor triple bond is to add bromine water to the compound.• If a double or triple bond is present, the bromine waterchanges colour from yellow–brown to colourless. • This reaction, shown in Figure 1020a, which also occurs with chlorine and iodine, takes place spontaneously at room temperature and pressure.

CHEMICAL PROPERTIES OF ALKENESCHEMICAL PROPERTIES OF ALKENES

The addition of bromine dissolved in tetrachloromethane (CCl4) or water (known as bromine water) is used as a test for unsaturation. If the reddish-brown colour is removed from the bromine solution, the substance possesses a C=C bond.

ELECTROPHILIC ADDITION OF BROMINE

TEST FOR UNSATURATION

PLACE A SOLUTION OF BROMINE IN A TEST TUBE

ADD THE HYDROCARBON TO BE TESTED AND SHAKE

IF THE BROWN COLOUR DISAPPEARS THEN THE HYDROCARBON IS AN ALKENE

A

B

C

A B C

Because the bromine adds to the alkene, it no longer exists as molecular bromine and the typical red-brown colour disappears

Catalytic Hydrogenation

• A similar spontaneous reaction occurs between alkenes and hydrogen halides such as hydrogen chloride and this is shown in Figure 1020b.

• With hydrogen, the activation energy is slightly higher, but if a gaseous mixture of an alkene and hydrogen is passed over a heated nickel catalyst, an addition reaction to form analkane occurs as shown in Figure 1020c.

• This reaction is the basis of the conversion of vegetable oils, which contain a number of C=C double bonds, into margarine, which has fewer double bonds and hence a higher melting point.

Polymers• Polymers are long chain molecules that are formed by

the joining together of a large number of repeating units, called monomers, by a process of polymerisation.

• Polymers,can be made artificially and these are usually referred to as plastics, but there are also a great number of naturally occurring polymers.

• One type of polymerisation reaction is known as addition polymerisation. In this the monomers contain double bonds and in the addition reaction new bonds (shown coloured below) form between these monomer units.

• The simplest polymerisation reaction of this type is that of ethene when heated under pressure with a catalyst to form polyethene, commonly known as ‘polythylene’.

Polythylene

• polyethylene formation may also be represented by the equation below in which the repeating unit is shown in square brackets.

Polyvinyl Chloride (PVC)

Another common addition polymer is poly(chloroethene), better known as PVC (short for its old name of PolyVinyl Chloride), formed by the polymerisation of chloroethene

Monomer Polymer

Chloroethene Polyvinyl chloride (PVC)

Monomer Polymer

Tetrafluoroethene Polytetrafluoroethene (Teflon)

Polypropene

Polypropene is another common adition polymers.