Monthly Archives: November 2016

Benzene

Review

Discuss with your neighbour everything you remember about the structure of benzene.  Use the diagram below to jog your memory.

Listen to the Voice Thread about Benzene that you created last year.

Review your notes and the section in the text about the structure of benzene before continuing.

Electrophilic Substitution of Benzene

Despite the π-bonds, benzene does not undergo addition reactions like an alkene would.  It does however undergo electrophilic substitution.  

✍️  Define the term electrophile and give 3 examples.

Benzene is an electron rich molecule.  This makes it susceptible to attack by electrophiles.  It will react with a mixture of concentrated nitric and sulfuric acids to form nitrobenzene.

Mechanism for the nitration of benzene (HL only)

✍️  Use your text book (and any other sources you need) to make a complete summary of electrophilic substitution.

Reaction pathways (HL only)

Add the nucleophilic substitution reactions and electrophilic substitution of benzene on to your map.  Remember to add as much detail about conditions as you can.

Halogenoalkanes

Reactivity

Halogenoalkanes are more reactive than alkanes.

Reveiw – Why are alkanes unreactive?  If you can’t answer this question, you need to review alkanes.

✍️  Draw and name all of the isomers of C4H9Br and classify them as primary, secondary and tertiary.

What is different about halogenoalkanes that makes them more reactive than alkanes?  Consider the two points that make alkanes less reactive.  Is there any differences with halogenoalkanes?

Nucleophilic Substitution Reactions of Halogenoalkanes

✍️  Define a nucleophile and give 3 different examples.

We are only going to be concerned with using hydroxide ion (OH) in aqueous solution as our nucleophile.

✍️  (HL) Why is hydroxide ion a better nucleophile than water?  Check and see if you agree with the video below:

When a halogenoalkane is reacted with aqueous OH– an alcohol is produced.

screen-shot-2016-11-14-at-6-21-56

 ✍️  Draw and name the products of nucleophilic substitution with all the isomers of C4H9Br you drew earlier.

We have finished with the standard level material in this section.  Time for review!

Review questions              Review Answers

Nucleophilic Substitution Mechanisms (HL only)

Depending on whether the halogenoalkane is primary, secondary or tertiary, depends on the mechanism for this reaction.

Primary halogenoalkanes tend to react via a SN2 mechanism.
Tertiary halogenoalkanes tend to react via a SN1 mechanism.
Secondary halogenoalkanes use either and you can’t predict which one.

SN1 Mechanism
screen-shot-2016-11-15-at-6-49-32
SN2 Mechanism

screen-shot-2016-11-15-at-6-35-28

 

Examine the two mechanisms.  They are written for any halogenoalkane and any nucleophile.

Key to the mechanisms:

L = leaving group.  This is the halogen F, Cl, Br or I.
Nu = nucleophile. This could be OH or any other species with a lone pair.

Now have a look at the following animation.  Here is the link if you want to see the original.
There is more than one type of mechanism here so choose unimolecular nucleophilic substitution for SN1 and bimolecular nucleophilic substituion for SN2.

✍️   After examining the mechanisms and the animation, try and answer the following questions:

  • What does S and N stand for in the notation of the mechanism (SN2)?
  • The numbers 1 and 2 stand for the molecularity of the mechanism.  What does this mean?
  • Define the terms unimolecular and bimolecular.
  • Which mechanism has a carbocation intermediate?  Identify it.
  • Which mechanism forms a transition state?
  • What are the coloured arrows trying to indicate in the SNmechanism?

Conditions for the Reactions

SNreactions are best conducted using protic, polar solvents.
SNreactions are best conducted using aprotic, polar solvents.

Polar, aprotic solvents include:

  • propanone
  • N,N-dimethylmethanamide
  • ethanenitrile

Polar, protic solvents include:

  • water
  • ammonia
  • 2-methylpropan-2-ol
  • propan-1-ol and propan-2-ol
  • ethanol
  • methanol
  • ethanoic acid

✍️  Draw the structures for the two groups of solvents.
✍️  What is the difference between an aprotic and a protic solvent?

Rate of Reaction

Examine the two mechanisms again.

✍️  Write a rate equation for each mechanism.

How does the type of halogen affect the rate of reaction?

✍️  Using your data booklet fill in the electronegativities and bond enthalpies for the difference carbon-halogen bonds.

C-X             Electronegativity              Bond enthalpy

C-F
C-Cl
C-Br
C-I

✍️  What trends to you notice?  Discuss these with your table.
✍️  Which halogenoalkane would a nucleophile be most attracted to?

Despite the polarity of the bonds, the most important factor in determining rate is bond strength.

✍️  Knowing this, rank the halogenoalkaness in order from fastest to slowest for reaction with a nucleophile.

Everything you need to know about these two mechanisms is summarised on this sheet here.
✍️   Before trying the review questions, read the relevant section in your text and annotate your notes with any extra important information.

Review Questions              Review Answers

Reduction Reactions (HL)

Just as you can oxidise alcohols to form compounds with a carbonyl group, you can then reduce carbonyl containing compounds back to alcohols.

Reagents for reduction

  • Lithium aluminium hydride                 LiAlH4
  • Sodium borohydride                             NaBH4

Lithium aluminium hydride

Sodium borohydride

 

Structures of the reducing agents

 

 

Read through your text and annotate your notes find the answers to the questions below.

✍️  What difference is there in the conditions under which the two reagents might be used?
✍️   Which reagent is preferred from the reduction of carboxylic acids and why?

Reduction of Carbonyl Containing Compounds

screen-shot-2016-11-11-at-6-54-38

✍️  For the following starting materials, draw the structures and name to products formed when treated with lithium aluminium hydride.

  • propanoic acid
  • butanone

Conversion of nitrobenzene to phenylamine

This reduction happens via a two step process as summarised below.

✍️   After reading your text or other sources, elaborate on what is happening in both stages in your notes.  Make sure you can answer these questions.

  • Why is a protonated phenylammonium ion produced in the first step?
  • What technique is used to heat the reaction?
  • What is the role of the tin?
  • What is the purpose of using sodium hydroxide in the second stage?

Alcohols

Alcohols as fuels

The use of ethanol as a fuel is growing around the world. It is hailed as a more environmentally friendly fuel than fossil fuel because the carbon dioxide released from burning the fuel was what the crop absorbed whilst it was growing meaning that no new carbon dioxide has been added to the atmosphere.

Can you see a problem with this logic?  Take a look at the cycle of ethanol production and use below.  How ‘green’ is ethanol as a fuel?

Production and Use of Ethanol as a fuel

There has been a lot written about ethanol as an alternative fuel.  If you’re interested, here are a couple of articles with more information:

Corn Biofuel Dangerously Oversold – New Scientist
Can Ethanol from Corn be Made Sustainable – Scientific American

The complete combustion of ethanol is as follows:

C2H6O(g) + 3O2(g) –> 2CO2(g) + 3H2O(g)

✍️   Write equations for the complete combustion of methanol, propanol and butanol.

Oxidation of Alcohols

1.  Primary, Secondary and Tertiary Alcohols

✍️   Draw the structures and name all the alcohols with molecular formula C4H10O.
✍️   Classify these into primary, secondary and tertiary alcohols.

2.  Common Oxidising Agents

In the next unit (Topic 9/19 of your syllabus) we will discuss these in more detail.  However, for now, we will look at two reagents that are used for oxidising alcohols:

  • acidified potassium permanganate (VII)       KMnO4
    OR
  • acidified sodium dichromate (VI)                   Na2Cr2O7

Either of these two reagents can be used.  It is important to learn what their colours before and after reaction.

screen-shot-2016-11-10-at-20-15-39

3.  An Experiment

A student decided to look at what types of alcohols were able to be oxidised.  She decided to use the following alcohols:

  • ethanol
  • propan-1-ol
  • propan-2-ol
  • 2-methylpropan-2-ol

✍️  Draw the full structural formula for each of the alcohols above.
✍️  Classify them as either primary, secondary or tertiary.

She decided to try reacting the alcohols with acidified sodium dichromate(VI) in one trial and acidified potassium permanganate(VII) in the other.  She set up the two trials as shown below with these reagents.

Oxidation of alcohols

Acidified potassium permanganate(VII) BEFORE reaction with alcohols.

Oxidation of alcohols

Acidified sodium dichromate(VI) BEFORE reaction with alcohols

Into the wells, she put two drops of the following alcohols:

A1 Ethanol
A2 Propan-1-ol
A3 Propan-2-ol
A4 2-methylpropan-2-ol
and
B1 or B2 – no alcohol as this was the control

After 15 minutes, she observed the following changes.

Oxidation of alcohols

Acidified sodium dichromate(VI) AFTER reaction with alcohols

Oxidation of alcohols

Acidified potassium permanganate(VII) AFTER reaction with alcohols

✍️  From her results, which types of alcohols (primary, secondary and/or tertiary) undergo oxidation?
✍️  The tray with the potassium permanganate(VII) showed a reaction but a brown precipitate formed in the wells.  What is this?

For now we aren’t going to worry about trying to balance these redox equations but instead just focus on what happens to the alcohol.

This will depend on whether the alcohol is primary secondary or tertiary.  Below is a diagram representing the different possibilities for the oxidation of alcohols.

screen-shot-2016-11-10-at-20-35-49

✍️  After examining the chart, what were the products of the reactions in each of the wells A1, A2, A3 and A4?

4.  Techniques for Oxidising Alcohols

Heating under reflux

Distillation

 

Techniques for heating and recovering products in organic chemistry.

 

 

 

 

 

✍️  Using the above chart, what would be the products when the following are oxidised under the conditions specified:

  1. Butan-1-ol is reacted with stoichiometrically equivalent amounts of acidified potassium permanganate (VII) and the product is removed by distillation as it is formed.
  2. Methanol is reacted with excess acidified sodium dichromate (VI) and heated under reflux before the product is removed by distillation.
  3. Butan-2-ol is reacted with excess acidified potassium dichromate (VI) and heated under reflux before the product is removed by distillation.
  4. Methylpropan-2-ol is heated under reflux with excess potassium permanganate (VII).

Esterification

Esterification is a type of condensation reaction where an alcohol and a carboxylic acid are combined to form an ester.

screen-shot-2016-11-10-at-20-57-14

Some important points to note are:

  • this is a reversible reaction so a 100% yield is impossible to obtain
  • reaction requires heat
  • reaction requires an acid catalyst usually in the form of concentrated sulfuric acid
  • esters are often fragrant and many have fruity smells

✍️   Write the equation (using structural formula for all organic compounds) between ethanol and butanoic acid.  Name the ester produced.

Reaction Pathways (HL only)

So far we have talked about alkanes, alkenes and alcohols.  We have also made halogenalkanes, aldehydes, ketones, carboxylic acids and esters in our discussions.

✍️  Discuss at your table how you could make ethanoic acid from ethene.  What reagents would you need and under what conditions (heat, reflux, distillation) would you use at each step?

✍️  Construct a map that connects the types of compounds we have discussed so far.  Over the arrows, put the conditions and reagents needed for the reactions.

Alkenes

Reactivity

Alkenes are more reactive than alkanes.  Electrons in π bonds are not as strongly attracted to the nuclei as the electrons in the σ bond.  This makes the π bond weaker.

✍️    HL – What is the hybridisation of carbon in this molecule?  If you can’t answer that question, you need to revise this section from bonding.

A model of ethene showing the electron distribution

Distinguishing between alkanes and alkenes

✍️   Review this summary of alkenes and add your own summary to your notes.  Include an example of the reaction that occurs when adding bromine water to an alkene.

✍️  What type of reaction is occurring in the alkene test tube?

✍️   If left overnight, the test tube containing the alkane would also decolourise.  Write a set of equations to explain this and identify the type of reaction occurring.

Addition reactions

Alkenes can react with:

  • halogens eg F2, Cl2, Br2 and I2
  • hydrogen halides eg HF, HCl, HBr and HI and
  • water H2O in the presences of a sulfuric acid catalyst
  • hydrogen in the presences of a Ni catalyst

✍️  Write the equation for the reaction of but-2-ene with each of the following reactants above.  Use structural formulae in the equations and name the product each time.

✍️  What classes of compounds can be made from addition reactions of alkenes?

Addition Polymerisation

View the video found on this page.

Review pages 3 and 4 of this summary and write your own notes.  Make sure you view both the animations.

Key terms to remember here are monomer and polymer.  Make sure you can define both and know how one relates to the other!

Try the polymer puzzles found here.

Alkenes are useful compounds.  You can read about them here.

This concludes the material for standard level.  You should now read the section in your text book (10.2) which is relevant to alkenes and addition polymerisation and add any thing else you find important to your notes.

Practice problems   and    solutions

Electrophilic Addition Reactions – HL only

Study the following image of a general mechanism for electrophilic addition reactions.  What do you think it is showing you?  Think about the following:

  • What do you think the curly arrows are representing?
  • What does r.d.s stand for?
  • Why is the second step faster than the first step?

The above diagram is the general mechanism for electrophilic addition of any halogen (X2), halogen halide (HX) or interhalogen (eg I-Cl or iodine monochloride) with any alkene.

✍️  Draw the mechanism for the bromination of ethene.

Remembering that an electrophile is an electron deficient species, how is Br2 considered an electrophile in this mechanism?

Below is the reaction between propene and HBr.

As you can see, there are two possible products.  One is more likely than the other.

✍️  Draw the mechanism to create both the products.

The Markovnikov rule explains why 2-bromopropane is the major product.  Simply put the Markovnikov rule is…

“The hydrogen rich get richer!”

✍️  Use your textbook to read about this rule and write a summary to explain why the major product is 2-bromopropane and not 1-bromopropane in terms of the stability of the carbocation.

Practice problems  and  solutions

Post Expedition Reflection

Write a blog post about your experience on expedition.Red tree #yis7xp

As a guide, reflect on the following questions:

  • What did I enjoy?
  • What did I do well?
  • What challenges did I face?
  • What did I learn about living and working in a community?
  • How did I grow?
  • Why are the Expeditions an important part of our schooling?
  • What skill/idea/goal that I began to work on during the expedition, do I want to continue with at school this year?

You might like to include some of the photos taken by the teachers to illustrate your post (try and include relevant images) or include some of your own.