Organics - alkenes Flashcards
What does alkenes being ‘unsaturated’ mean?
Alkenes have at least one double bond. They do not have the maximum number of hydrogen’s, so they aren’t fully saturated with hydrogen’s and therefore can undergo addition hydrogenation to become fully saturated. Cycloalkenes and alkenes are unsaturated.
C=C double bonds
A double bond is a region where both sigma and pi orbitals coexist.
When there is one sigma bond and one pi bond this forms a double bond however if there is one sigma bond and TWO pi bonds this forms a TRIPLE BOND - ‘Comprised of one sigma bond and two pi bonds between two particular atoms in a molecule’
Sigma bonds (single bonds)
Comprised of a sigma bond
There are 3 main ways sigma bonds are formed. They can form from the overlap of either of these:
1) 2 p orbitals (head on horizontal eight shapes joined-dumbbell shape)
2) 2 s orbitals (two circles joined together like a venn diagram)
3) 1 s orbital and 1 p orbital (the horizontal eight shape joined with a circle)
Sigma bonds are very strong. They are covalent bonds formed by head on overlap of orbitals.
Pi bonds
There is only one main way that a pi bond can form:
1) 2 p orbitals (two eight shapes stand vertically and overlap sideways) - ‘formed by the sideways overlap of atomic P orbitals which results in regions of high electron density above and below the plane of nuclei of the binding atoms’
Pi bonds are very weak. When we break a double bond we are essentially breaking the pi bond to leave a sigma bond (which is the single bond).
Electrophile
A species which is an electron pair acceptor. It is an electron deficient (lacking e-) species that is attracted to an area of high electron density/negative charge. They can be positive but they don’t have to be.
Addition reaction of alkenes with hydrogen to form an alkane
This reaction is called a [catalytic] hydrogenation reaction: a type of addition reaction whereby the hydrogen is added to the compound, causing the double bond to break and form a single bond.
Conditions: nickel catalyst (might also be platinum)
Hydrogen is a positively charged proton that is an electron acceptor; it wants to gain an electron to fill its outer shell and stabilise. Therefore it is attracted to the double bond of an alkene as this is an area of high electron density/negative charge. The hydrogen undergoes electrophilic attack on the double bond where hydrogen is the electrophile. This then forms a single bond and so an alkane. This is a type of electrophilic addition and because it involves hydrogen it is a ‘hydrogenation’ reaction.
Mechanism:
1) Hydrogen adsorbs onto the the metal catalyst (nickel) which causes the bond between the hydrogen atoms in the molecule to break [homolytic fission], then this produces two identical hydrogen atoms.
2) the alkene in the reaction undergoes electrophilic attack by the electrophile which in this case is the hydrogen. This occurs as adsorption where the hydrogen atom is adsorbed into the alkene.
3) This is a chain reaction as once the first hydrogen has been added in it promotes addition of the second hydrogen atom. It follows on and is easily added to the alkene because less energy is required (due to the pi bond already being broken).
Margarine is made through the hydrogenation of unsaturated vegetable oils such as sunflower oil
Addition reaction of alkenes with hydrogen halides to produce halogenoalkanes - MECHANISM
For example, propene reacting with hydrogen bromide
1) H-X : Halogen (X) is more electronegative than hydrogen so it pulls the electrons they are sharing towards itself creating a dipole. The hydrogen becomes partially positive and the bromine becomes partially negative.
2) The electrons in the double bond of the alkene are attracted to the partially positive hydrogen (CURLY ARROW)
3) Hydrogen gains these extra electrons, bromine is more electronegative so it pulls these extra electrons towards itself making the dipole stronger until the bond is weak and breaks. A hydrogen ion is then free. It is attracted to the C=C because that is an area of negative charge. This means a carbocation forms because the alkene has essentially gained a proton
[Markovnikov’s rule if unsymmetrical alkene: the hydrogen attaches to the carbon in the C=C which already has the most Hydrogen’s on it. The + goes on the other carbon]
4) 50% primary and 50% secondary carbocations form however primary carbocations are least stable and therefore break down meaning that bromine is more abundantly attracted to the secondary carbocations which form the MAJOR product. The primary carbocations which have bromide attracted to them form the MINOR product.
NOTE: The breaking of the H-Br bond is heterolytic fission. The electrophile is H+. The reagent is HCL or HBR. (Check if a common exam Q is why only these two is it something to do with reactivity?)
Addition reaction of alkenes with halogens to produce dihalogenoalkanes - MECHANISM
For example, reacting ethene with bromine
1) the pi bond in ethene causes a dipole to be induced in the Br-Br (this is shown by the SMALL CURLY ARROW)
2) the electrons from the C=C bond are attracted to the delta positive Br in the Br-Br bond, this induces a stronger dipole causing the bond to weaken and then break. It splits by homolytic fission and produces two bromine ions.
3) The delta positive brominehas attached to the carbon on the alkene. The alkene forms a positive charge on the other carbon because the double bond broke and so that carbon is one electron short.
[Originally it had 4 bonds but now it has three bonds so - it is one electron short because the double bond is broken but the other carbon isn’t short because even though the double bond broke it still has 4 bonds bc it formed a new bond with the bromine which has a positive charge but the carbon with the positive charge didn’t make any new bonds to compensate so it has lost an electron]
4)This forms a carbocation and a separate negative bromine atom, this Br then attracts to the positive carbon and a DIHALOGENOALKANE forms
Stability of carbocations
The least stable carbocations will be broken down
Where > means more stable:
Tertiary>Secondary>primary
Primary is least stable as it only has one alkyl group and the more alkyl groups means it is more stable
The most stable carbocation will form the major product and the least stable carbocation(s) forms the minor product
Addition reaction of alkenes with potassium manganate (VII) to produce a diol
(what would propene produce, temperature, type of agent, what happens with the C=C, colour change, ions, reaction as a test for…)
For example, propene producing propene-1,2-diol (acidic conditions)
Propene reacts at room temperature with acidified potassium manganate (VII).
The acidified potassium manganate is an oxidising agent when it is in the presence of acid, it oxidises the double bond to a single bond and also provides a water molecule to produce the two OH groups which come off of the two Cs in the C=C bond.
- react in the cold
- The purple colour of the MnO4- ion also decolourises to colourless
- manganate (VII) ions reduced to manganese (II) ions
- oxidation reaction.
This reaction can therefore be use to distinguish between alkenes and alkanes as if it is an alkane the ion WON’T become colourless.
Addition reactions of alkenes with steam to produce alcohols
two types of reaction, three conditions
For example, ethene and steam produces ethanol. The OH and H from the water molecule bind to the ethene after its double bond is broken, producing the alcohol. This is a hydration reaction.
The conditions for this are with a phosphoric acid catalyst, 60-70 atm and and 300-600 degrees.
REVERSIBLE REACTION
NOTE: You can also dehydrate alcohols to produce an alkene
Markovnikov’s rule: the hydrogen from the H2O adds to the carbon with the most hydrogens already, then the OH adds to the other carbon.
Bond Fission
Heterolytic bond fission - the molecule breaks up into two unidentical ions
Homolytic bond fission - the molecule breaks up into two identical ions (Eg. Free radical substitution when two radicals are produced Br2 —> Br dot
Addition reaction of alkenes with hydrogen halides to produce halogenoalkanes - CONDITIONS
alkenes react with gaseous hydrogen halides at room temperature. (if alkene also a gas then mix gases, if alkene a liquid then bubble hydrogen halide through alkene)
Addition reaction of alkenes with hydrogen halides to produce halogenoalkanes - REACTION RATE
reaction rate increases from using hydrogen halides HF to HCl to HBr to HI
the more methyl groups and complex the alkene is, the more stable the carbocation it forms is, so the more readily reactive it is - (so the reaction rate increases as the alkene gets more complicated)
Addition reaction of alkenes with halogens to produce dihalogenoalkanes -CONDITIONS
Bromine and alkene specifically: react in the cold, bromine loses its brown colour to give a colourless liquid
