Alcohols and Organic Analysis

How are alcohols classified as primary, secondary or tertirary?

• Primary- Two (or three if methanol) H atoms bonded to the carbon carrying the OH group

• Secondary- One H atom bonded to the carbon carrying the OH group

• Tertiary- No H atoms bonded to the carbon carrying the H group

What conditions are required for the dehydration of alcohols?

• Alcohols undergo dehydration reactions through elimination reactions when heated with concentrated sulfuric acid at 450K

• Since water is eliminated, we refer to this as a dehydration reaction

What is a dehydration reaction?

A reaction where water is lost from a compound

What is the difference between condensation and dehydration reactions?

• Condensation reactions remove water to join molecules

• Dehydration reactions just remove water from a compound

What is the equation for the dehydration of ethanol?

CH₃CH₂OH (g) → CH₂=CH₂ (g) + H₂O (l)

H₂SO₄ above arrow

Outline and name the mechanism for the dehydration of alcohols?

(Acid-catalysed) elimination

• A bond forms as a lone pair of electrons on oxygen forms a bond with a H+ ion

• This forms an intermediate where the oxygen is bonded to two hydrogens and has a positive charge

• The pair of electrons in the C-O bond move to the positively charged oxygen, breaking the bond

• This forms a carbocation intermediate and water

• The electrons in one of the C-H bonds in the carbon adjacent to the positively charged carbon moves to the C-C bond forming a C=C bond and regenerating a H+ ion

• This forms an alkene and a H+ ion, so the H+ ion acts as a catalyst

What two ways can ethanol be made industrially?

• Direct hydration of ethene with steam

• Fermentation

What is the equation for the direct hydration of ethene with steam?

CH₂=CH₂ (g)+ H₂O (g) → CH₃CH₂OH (g)

Catalysed by phosphoric acid (show above arrow)

What conditions are needed for the direct hydration of ethene with steam?

• A phosphoric acid catalyst (H₃PO₄)

• 570K

• 6-7 MPa of pressure

Name and outline the mechanism for the hydration of ethene with steam

Electrophilic addition

• The H+ ion (an electrophile) is attracted to the dense region of negative space in the C=C bond

• The double bond breaks, forming a C-H bond and a carbocation intermediate

• Water then reacts with this carbocation to from a C-O bond with two hydrogens bonded to the oxygen, making the oxygen positive

• One of the O-H bonds breaks as electrons move to the positive oxygen

• This forms an alcohol and a H+ ion, so the H+ ion acts as a catalyst as it is regenerated

Why are high temperatures and pressures used for the direct hydration of ethene with steam?

To prevent the reaction from moving backwards

How does industrial fermentation work?

• Sugar cane, molasses or starch is broken down into glucose for fermentation

• Zymase in yeast catalyses fermentation

• The product is a mixture of water and about 15% ethanol by volume

What conditions are needed for industrial fermentation?

• Aqueous glucose

• Yeast catalyst and 310K

• 100 KPa

• Anaerobic conditions

What is the equation for fermentation?

C₆H₁₂O₆ → 2CH₃CH₂OH + 2CO₂

What are the advantages of direct hydration of ethene with steam for the production of ethanol?

• Produces more ethanol

• Faster

• No waste products

• 100% Atom economy

What are the disadvantages of direct hydration of ethene with steam for the production of ethanol?

• High temperature and pressure = money

• Ethanol is produced in the gaseous state

• Depends on non-renewable resource- ethene

What are the advantages of industrial fermentation for the production of ethanol?

• No high temperatures or pressures- less money

• Depends on renewable source- yeast

• CO₂ can be used as a feedstock

What are the disadvantages of industrial fermentation for the production of ethanol?

• Only produces 15% ethanol by volume

• Waste product of CO₂ increases carbon emissions

• Temperature has to be kept constant for yeast enzymes

• Slower

• A lot of natural materials like sugar cane or potato needed

• Yeast sensitive to temp, pH

• Requires a substantial amount of water

How can primary alcohols be oxidised and what are they oxidised to?

Primary alcohols are oxidised to aldehydes by and oxidising agent called potassium dichromate (VI) which is acidified (K₂Cr₂O₇/H⁺)

What is oxidation?

• Gain of oxygen

• Loss of electrons

• Loss of hydrogen

What is reduction?

• Gain of electrons

• Loss of oxygen

• Gain of hydrogen

How do we show oxidising agents because chemists are lazy?

[o]

Outline the equation to show the oxidation of ethanol?

• Draw out ethanol then + [o)

• Draw out ethanal + H₂O

What are the conditions for the oxidation of ethanol?

Takes place at room temperature

How are aldehydes separated when an alcohol is oxidised?

• By distillation

• When boiling, we break IMFs

• Different boiling points are used to separate substances

Draw out the labelled distillation apparatus

Why does the reaction of alcohol with an oxidising agent have to be distilled immediately?

To prevent further oxidation

What occurs during further substitution of the reaction of an alcohol with an oxidising agent?

Further oxidation of the aldehyde would produce a carboxylic acid

How can we get straight from an alcohol to a carboxylic acid?

• Use excess oxidising agent (2x)

• Heat under reflux

Draw and label reflux apparatus

What happens during reflux?

• The aldehyde is formed and condensed back into the reaction mixture when vapourised

• This forces the aldehyde to be oxidised further

How can primary, secondary and tertiary alcohols be oxidised?

• Primary alcohols can be oxidised into aldehydes by acidified potassium dichromate (VI)

• Secondary alcohols can be oxidised into ketones by acidified potassium dichromate (VI)

• Tertiary alcohols cannot be oxidised further as the carbon that carries the OH bond is bonded to no hydrogens

How do we test for alcohols?

• Add aqueous potassium dichromate (VI) acidified by dilute sulfuric acid

• Solution turns from orange to green

What is a limitation of a test with alcohols

We cannot identify tertiary alcohols

Why does a solution turn from orange to green when testing for an alcohol?

• Cr₂O₇^2- is reduced to 2Cr³⁺

• Cr₂O₇^2- is orange and 2Cr³⁺ is green

What tests can we use for aldehydes?

• Tollen’s reagent

• Fehling’s solution

How does Tollen’s reagent identify an aldehyde?

• Reagent is ammoniacal silver nitrate [Ag(NH₃)₂]⁺NO₃^-

• This acts as a mild oxidising agent

• When the aldehyde is oxidised, it forms a carboxylic acid

• Silver atoms form a silver mirror

What is the ionic equation to show the formation of a silver mirror

[Ag(NH₃)₂]⁺ + e^- → Ag_(s) + 2NH_{3 (aq)}

How does Fehling’s solution identify an aldehyde?

• Is a blue solution containing Copper (II) ions (Cu²⁺) in an alkaline solution (NaOH)

• Cu²⁺ are mild oxidising agents- aldehydes oxidised to form a carboxylic acid

• Aldehydes → Carboxylic acid + e^-

• Cu²⁺ ions are blue while Cu⁺ ions are red, so a brick red precipitate forms

Write the ionic equation for the formation of a brick red precipitate in Fehling’s test

Cu²⁺_(aq) + e^- → Cu⁺_(s)

How do we test for alkenes?

• Shake alkene with bromine water

• Orange solution becomes colourless

What test can we conduct to identify carboxylic acids?

• Pour some test solution into a test tube containing sodium carbonate solid

• Observe fizzing

What is the word equation that shows what happens when an acid and carbonate react?

Acid + Carbonate → Salt + Carbon Dioxide + Water

What is the equation for the reaction of ethanoic acid with Sodium carbonate?

2CH₃COOH + Na₂CO₃ → 2CH₃COONa (sodium ethanoate) + CO₂ + H₂O

How can we test for halide ions/halogenoalkanes?

• Add aqueous sodium hydroxide to a halogenoalkane and warm gently in a water bath- nucleophilic substitution occurs, releasing halide ions

• Add aqueous acidified silver nitrate

• Chloride ions form a white precipitate

• Bromide ions form a cream precipitate

• Iodide ions form a yellow precipitate

Why can 2,2-dimethylpropan-1-ol not be dehydrated? (This is just and example)

• Because the carbon adjacent to the carbon carrying the OH group has no hydrogens attached

• This means that when the carbocation forms, there is no OH bond on the adjacent carbon that can be broken in order to form a double bond

What is high resolution mass spectroscopy?

• We can determine the m/z of a peak to several decimal places

• This allows us to distinguish between compounds with similar Mr but different empirical formula

What is infrared spectroscopy?

• IR spec allows us to determine what functional groups are within a compound

• Any wavelength absorbed by the sample being examined will for a peak in a graph which can be used to determine the functional group

• Different bonds absorb IR at different frequencies

What is the wave number of a bond affected by in IR spec?

• The bond enthalpy

• The masses of the atoms

What is the fingerprint region?

• Region below 1500-400 wavenumbers

• Allows experienced chemists to determine the exact compound

• It is unique in every compound

• Sample spectrum is compared with database

What does the peak for O-H in alcohols looks like?

• Broad peak

• Occurs at 3230-3550 (above 3000)

What is the peak for C-H bonds shaped like?

• Split into a few jagged peaks

• Occurs at 2850-3300

What does the peak of an O-H bond in carboxylic acids look like?

• Less smooth and more broad than O-H in alcohols

• Occurs between 2500-3000

• There is also a small peak on the side showing a C-H bond

What does a C=O bond look like?

• Long, pointy and not jagged

• Occurs between 1680-1750