Chemistry- Unit 2- Alkenes/Alcohols

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  • Created by: FireDwarf
  • Created on: 18-03-14 21:14
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What is the shape of a alkene like? Its angles?
It is a planar molecule. Has its angles between bonds at 120 rouhgly.
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What about if the alkene is larger?
Only the area around the C=C is planar.
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Why is there no rotation around the C=C bond? What does this cause?
Because there is a pie and sigma bond. Pie is produced from the 2 p orbitals from each carbon merging. This stops rotation. Causes E and Z isomers.
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What are E and Z isomers? How do we decide which one is if we have 3 functional groups?
Geometrical isomers- and if we have 3 groups the heavier ones should be opposite/on the same side.
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What is geometric isomers an example of? Definition?
Stereoisomerisation & they are molecules with the same structural formula but atoms aranged diffrently in space.
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Physical properties of alkenes?
Just van der waals. Properties like alkanes. Melting point and boiling increase with carbon chain and branching. Not soulable in water.
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Reactivity? despite higher bond enthalphys
are more reactive then alkanes because of the double bond. This is because it creates an high electron density in the molecule which attracts electrophiles.
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What are these reactions known as?
Electrophillic additions.
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Reaction mechanisim for electrophile addition?
1st arrow goes from the C=C bond to the electrophile's positve charged atom (eg H+). Second arrow goes from the bond between it and the halogen. SECOND STAGE ---> Arrow from lone pair on halide to positive carbon.
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What is the second stages product called? What is the final product?
A carbocation & it forms a haloalkane.
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What happens if its a asympetrical alkene?
Because the double bond is not in the exact middle, the bromide can bond with multiple carbons. Therefore, multiple isomers can be formed.
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How do we work out which one is formed in larger proportions?
Based upon its structure. The more carbons it has bonded, the higher its structure is (primary,secondary tertiary) and therefore the more electrons it gives out. This makes it more stable because it stabalises the carbocation.
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What is this known as?
Positive inductive effect.
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What is the reaction mechanisim when it reacts with a halogen (Br2 eg).
There is a slight + and slight - on the Br-Br due to the random movement of electrons. 1st arrow from the C=C bond towards the Br. Second arrow from the Br-Br bond towards the negative Br.
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Second stage?
Left now with a bromide and a carbocation. 1st arrow from the Bromides lone pair towards the positive carbon atom.
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What is the test for a double bond?
Add a few drops of bromide solution (bromide water) which is redish-brown to the alkene. Will decolourise as the product formed is colourless.
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Reaction mechamisim for a alkene with sulfuric acid?
DRAW your sulfuric acid with a H-O-So3H. 1st arrow from the C=C to the positive H. Second arrow from the H-O bond towards the O-.
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Second stage?
From lone pair on the O towards the carbocation C+. Frorms (ethyl) hydrogensulphate
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What is the reaction with water? What can the acid be desribed as therefore?
When the product reacts with water, an alchol is produced and the sulfuric acid has regenerated. Therefore, it has acted as a catalyst for the reaction.
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What is addition polymerisation?
When monomers (repeated units) are addited together to make a longer chain of repeated units (a polymer)
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What does a monomer alkene form?
An alkane. It is however not called an alkane, but rather simply "poly" is added in front of the name of the monomer.
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How do we draw the monomer? How do we draw the polymer?
Bracket around the atoms in the monomer, with an N in the center on the left. Polymer has bonds going through the brackets, with n on the bottem right.
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What is a Low density polythene? HDPE?
Polythene is polyethene. Low density ones are flexible and strechy. HdPE are more rigid (bottles).
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Problems with Plastics?
Because they are essentally alkanes, and therefore very unreactive, they are not biodegradable. This causes large landfills where they remain for hundreds of years, and can even get into the ocean.
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Two solutions?
Mechanical recyling & feedstock recycling
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What is mechanical recycling?
Seperating the diffrent types of plastics, washing them, grinded into pellets, then heating and melted. Can be then used for diffrent purposes.
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The polymer can be heated to a temperture to break the bonds between the polymer and produce monomers, which can be used to make new plastics.
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Problems with both?
at each melting, the chain becomes shorter. Therefore, can only be done so many times. Each time desgenerates the plastics properties.
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What is the general formula?
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What is the shape around the O atom?
Because the oxygen has 2 lone pairs, the bond angle between the O and its H is 105.
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Is the melting and boiling point higher in alcohols then alkanes? any other effects?
Yes, the presence of a Oxygen convanently bonded to a hydrogen causes hydrogen bonding to be present between molecules. Also causes them to be souluble.
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How do we make ethanol from crude oil?
Thermal cracking of crude oil produces ethene. Ethene can be hydrated (exposed to steam) with a catalysit of phosphoric acid on silica. (C2H4 + H20 ---> C2H5OH)
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What is the proccess where sugar is broken down into ethanol? What is the catalyst? What does it rely on?
Fermentation, yeast, anerobic resperation of the yeast.
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C6H12O6 ---> (Yeast) 2C2H5OH + 2Co2
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How is the rate of the reaction effected?What needs to be kept out? What is the percentage where it must stop?
Temperture, slow at low temertures but going too high denatures the enzyme. Air to prevent aerobic resperation. 17%
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What does carbon-neutral mean?
The Co2 released from being burnt is balanced by the carbon dioxide it absorbed (plant) when it was originally obtained.
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Comparing the useage of ethanol production via fermentation compared to hydration of ethene. (ETHENE PROS)
Its fast, high yield, 100% atom economy, contineous, not much land needed.
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Fermentation Pros.
Renewable, low pressure and low temperture. Argued to be carbon neutral. Only a yeast catalyst.
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Why could fermentation not be carbon neutral?
Because of various transports needed to carry it where it needs to go, each using fuel and therefore releasing Co2.
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Rnewable source of ethene?
dehyrating ethanol to produce ethene. Ethanol gained from fermentation of glucose.
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What is complete oxidisation?
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How else can it be completed?
Gently and in stages.
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When primary alcohols are oxidised originally, what do they produce? If we oxidise x further, what can we produce?
An aldehyde (+ H20) and then a carboxyllyic acid.
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If ethanol is a aldenhyde, what is its name? If its a carboxylic acid, whats its name?
ethanal & ethanoic acid
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When secondary alcohols are oxidised, what do they produce? What when they are oxidised further?
Ketones. Ketones are not oxidised further.
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What does propanol name in a ketone?
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Not oxidised easilly. This is because a C-C bond would need to bresk rather then a C-C,
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What solution is often used to oxidise primary and secondary alcohols?
Potassium dichromate which has been acidifided with dilute sulfuric acid.
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How does it work?
The dichromate orange ions (6) get reduced (as they are the oxidising agent) into green chromate (3) ions
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Oxidising ethanol to ethanal? Experiment details?
Use a flask with ethanol and excess dichromate ions (with concentrated sulfuric acid). Apply heat at the flask .Have continious tube going upwards with NO interuptions. Input for water in, output for water.
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CH3Ch2OH + [O] ---> Ch3CHO + H20
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What is this proccess known as ? What does it mean?
Reflux- Vapour condenses and drips back into the reaction flask
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How do we get carboxylic acid?
We complete relfux like the other method, so that any ethanol or ethanal vapour condenses back into the reaction mixture. But, at around 20 mins we distill off the acid using a condensor.
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Ch3Ch2OH + 2[O] ---> Ch3COOH
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Diffrence between ketones and aldehydes?
They both have double bonds but aldehydes are at the end of the hydrocarbon chain while ketones has theirs in the center
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Name of the two tests for aldehydes and ketones?
Tollens (silver mirror) test & benedicts test.
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How to complete the silver mirror test?
Add a small amount of the substance being tested to tollens reagent (silver nitrate in aqeuous ammonia) will reduce colorless silver ions to a metallic silver. When heated, deposit of metallic silver is formed on the inside of the test tube.
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Turns blue copper sulphate ions into a brick red precipitate when heated.
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How can alcohols be dehydrated?
Using excess hot sulfuric acid or passing their vapours over heated aluminium oxide.
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What is formed? How can we form isomers?
A OH group is removed from the alcohol as well as a hydrogen to form H20 and an alkene. E-Z present if the OH is in the middle of the molecule & posistional isomers.
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Card 2


What is the shape of a alkene like? Its angles?


It is a planar molecule. Has its angles between bonds at 120 rouhgly.

Card 3


What about if the alkene is larger?


Preview of the front of card 3

Card 4


Why is there no rotation around the C=C bond? What does this cause?


Preview of the front of card 4

Card 5


What are E and Z isomers? How do we decide which one is if we have 3 functional groups?


Preview of the front of card 5
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