Addition Reactions of Alkenes Flashcards
what is a C=C bond made up of
a sigma bond and a pi bond
which bond is stronger and why
- the sigma bond is stronger
- because it is held more tightly between the carbon atoms
with that information does that mean that a double bond is twice as strong as a single bond and why
- no, it is between 1 and 2 times stronger
- because every single bond is a sigma bond which is stronger than a pi bond
- so if you have a sigma bond and a pi bond like a C=C, it will be stronger but not twice as strong
what do most reactions of alkenes generally involve in terms of the bonds
a double bond becoming a single bond
what bonds change during an alkene addition reaction
- the sigma bond remains unchanged
- but the electrons in the pi bonds are used to form new bonds with an attacking molecule
what is the product that is formed compared to an alkene if it only has single bonds
it is saturated
why is the product more stable than the alkene
- because the product only has sigma bonds, not pi bonds
- meaning the overall bonds in the product are stronger
what is the equation for the addition reaction between ethene and bromine
C2H4 + Br2 = C2H4Br2
what is that reaction generally used for
the chemical test for the presence of C=C in a compound
why is that reaction used as a chemical test
- because one of the reactants has a colour (bromine water)
- but the product is colourless
what is the correct way to describe what happens to bromine in the reaction when it reacts and the product is colourless
you say the bromine has been decolorised
what is hydrogenation
the addition reaction between an alkene and hydrogen
what would be the equation for the hydrogenation of ethene
C2H4 + H2 = C2H6
what will the the product of a hydrogenation reaction usually be and why
- an alkane
- because the product only has single bonds
- and has the general formula of CnH2n
what are the two things that are required in order for a hydrogenation reaction to occur
- heat
- a nickel catalyst
what are hydrogenation reactions practically used for
making margarine
how is margarine formed from hydrogenation
- naturally occurring vegetable oils are unsaturated
- so they contain C=C bonds
- when they react with hydrogen, some of the double bonds become single bonds
- this changes the properties of the vegetable oil and converts it into a solid
- aka margarine
what are monosaturated, polysaturated and satruated fats
- monosaturated fats are molecules that only have one C=C bons
- polysaturated fats are molecules that have more than one C=C bond
- saturated fats are molecules with no C=C bond
what is a halogenation reaction
a reaction between an alkene and a halogen
what are the products of most halogenation reactiond
dihalogenalkanes
what would a reaction between ethene and bromine produce if one bromine were to be bonded with one carbon atom each
1,2-dibromoethane
what is hydration
- adding water
- specifically, adding H and OH to the two atoms in a C=C bond
how is a hydration react carried out
- heating an alkene with steam
- passing the mixture over a catalyst of phosphoric acid
what would a reaction between ethene and water produce
ethanol
what is the difference between hydrogenation and hydration reactions
- hydration reactions actually make useful products
- so they are therefore used in industry a lot more
what are hydrogen halides often and what do they form in addition reactions
- they are hydrogen bromide or hydrogen chloride
- and they form halogenalkanes
what is the structural formula and the name for the reaction between ethene and hydrogen bromide
- CH2=CH2 + H-Br = CH3-CH2Br
- bromoethane
what is a diol
a compound containing two OH groups
what is the oxidising agent in acid conditions and what acid is usually used
- potassium manganate (VII)
- sulfuric acid
what is the summation of this reaction
- an oxidation reaction
- followed by an addition reaction
how are two OH groups formed when ethene reacts with the solution and the potassium manganate
- the potassium manganate provides an oxygen atom (oxidising)
- the water in the solution provides another oxygen atom and two hydrogen atoms
- forming the addition of two OH groups across the double bond
what is the structural formula for this reaction
CH2=CH2 + [O] + H2O = CH2OH-CH2OH
what does the [O] represent
the oxygen supplied by the oxidising agent
what is the name of the product
ethane-1,2-doil
why can this reaction be used to distinguish between and alkene and alkane like bromine is
- potassium manganate is normally purple
- in this reaction it changes from that to colourless
- it only turns colourless if there is a double bond present for the reaction
- so it can only occur with alkenes
