Organic pt3 ( Alcohols and organic analysis) Flashcards
What is the general formula for alcohol?
CnH2n+1OH
What are the physical properties of alcohol?
- High melting and boiling points than alkanes = due to hydrogen bonding caused by OH
- Alcohols with short chains are soluble in water because the hydrogen bonding predominates
- Alcohols with long chains are insoluble in water because the hydrocarbon chain dominates
How is ethanol made from crude oil? (where is ethene from, how does it turn into an alcohol, draw mechanisms and state conditions)
Ethene is produced when crude oil fractions are cracked
ethene is hydrated, which mean that water is added across the bond
steam 300 degrees and phosphoric acid
Electrophillic addition
Name 2 ways ethanol can be produced
- Hydration
- Fermenation
Describe how ethanol is made from the fermentation
- Fermentation (exothermic reaction) is carried out by yeast in anaerobic respiration
- Yeast produces an enzyme that converts sugars into ethanol and carbon dioxide
- Ethanol formed is separated by fractional distillation
What is the chemical equation for the fermentation of ethanol?
What are the conditions of fermentation?
Warm (25 to 42 degrees)
Aqueous
Yeast
Absence of oxygen
Why is the yeast used in fermentation?
Because it provides a source of enzymes (biological catalysts) = speed up the rate of reactions
Compare the Hydration of ethene to fermentation (Rate of reaction, use of resources, process type, waste products)
- hydration is faster,
- Hydration is made from non-renewable sources (ethene is from the cracking of crude oil) vs Fermentation is made from renewable sources (e.g. grapes and sugar canes)
- Hydration of ethene is a continuous process vs Fermentation is a batch process (have to start a new batch once yeast dies)
- Hydration produces pure ethanol vs fermentation produces impure (further processing needed)
- Hydration requires high temperature and pressure vs moderate temperature for fermentation so less energy needed
- fermentation releases co2 as a waste
What is the definition of a biofuel?
a fuel derived or produced from renewable biological sources
What is the definition of carbon neutral?
an activity on which the net carbon dioxide emission to the atmosphere is zero
What are the reagents, conditions, and observations for oxidation to form an aldehyde from ethanol? Draw the mechanism for ethanol to ethanal.
Reagents: dilute sulfuric acid/ potassium dichromate or potassium permanganate
Conditions: distil, needs to be from a primary alcohol
Observations: color change from orange to green with potassium dichromate or purple to colorless with KMnO4
What are the reagents, conditions, and observations for oxidation to form a CARBOXYLIC ACID from ethanol? Draw the mechanism for ethanol to ethanoic acid.
Reagents: dilute sulfuric acid/ potassium dichromate or potassium permanganate
Conditions: reflux, needs to be from primary alcohol
Observations: color change from orange to green with potassium dichromate or purple to colorless with KMnO4
What are the reagents, conditions, and observations for oxidation to form a KETONE from alcohol? Draw the mechanism for ethanol to ethanoic acid.
Reagents: dilute sulfuric acid/ potassium dichromate or potassium permanganate
Conditions: reflux, needs to be from secondary alcohol
Observations: color change from orange to green with potassium dichromate or purple to colorless with KMnO4
Why do we need to distil for aldehydes?
- Aldehydes have a lower boiling than alcohols and carboxylic acid : hydrogen bonds vs dipole-dipole
- To prevent further oxidation into carboxylic acids
What is reflux and why is it used for aldehydes?
- allows mixture to be heated, but prevents the volatile components from escaping the reaction mixture (the condenser is placed above the reaction vessel)
- to further oxidise into carboxylic acids
What is the mechanism and conditions for dehydration? Draw the mechanism for ethanol to ethene
Elimination
excess hot concentrated sulfuric acid
When is it not possible to dehydrate alcohol?
when there is no hydrogen attached to adjacent carbon
What is Tollens’ reagent?
Colourless solution of silver nitrate dissolved in aqueous ammonia
What are the two tests for aldehyde?
Tollen’s reagent
Fehling’s test
Describe how to carry out the Tollen’s reagent test and what happens when Tollens’ reagent is heated in a test tube with an aldehyde and when it’s heated with a ketone
Tollens reagent is made first: add some silver nitrate solution to a test tube, then add a few drops of sodium hydroxide so that pale brown precipitate appears. Add a few drops of ammonia until the precipitate disappears.
Add the aldehyde or ketone to the test tube. Place the test tube in a hot water bath
Aldehydes: Ag ions in Tollens’ reagent are reduced to Ag atoms. Silver mirror forms
Ketone: no precipitate forms.
What is Fehling’s solution?
Blue solution of copper(II) ions dissolved in sodium hydroxide
Describe the test and what happens to Fehling’s solution when it’s heated with an aldehyde
Fehlings solution is added to aldehyde or ketone
Place the test tube in a hot water bath
Aldehyde: Copper(II) ions are reduced to brick-red precipitate of copper(I) oxide
Ketone: no reaction
Why can’t tertiary alcohols be oxidised by acidified potassium dichromate/permanganate?
Because there are no hydrogen atoms bonded to the carbon which is bonded to -OH
Describe what happens when a reaction mixture is refluxed and why it is necessary for complete oxidation to ethanoic acid (3 marks - exam question)
- the mixture of liquids is heated to boiling point for a long period of time (allows the reaction to take place without losing volatile reactants/ products)
- vapours cool in condenser + condense back to liquids + drip back into the reaction mixture
- allows for the further oxidation of ethanal to ethanoic acid
What do anti-bumping granules do?
Anti-bumping granules prevent explosive boiling by stopping bumping and therefore splashing, as they provide a large surface area for bubbles to form on.
prevent large bubbles
Use your knowledge of structure and bonding to explain why it is possible to separate ethanal during distillation
Hydrogen bonding in ethanol and ethanoic acid
Ethanal has permanent dipole-dipole forces between molecules
dipole-dipole are weaker than hydrogen bonding;
Draw a reflux diagram
(the flask needs to be pear shaped)
Also its a Liebig condenser
Draw a distil diagram
make sure you label ethanol, (in the conical flask). Thermometer doesn’t touch the liquid because it’s measuring the gas
IMPORTANT: Thermometer should be at the T Junction
Describe how a student could use chemical tests to confirm that the liquid contained ethanal and did not contain ethanoic acid
Reagent to confirm the presence of ethanal:
- Add Tollens’ reagent
- Result with Tollen’s reagent: Silver mirror
OR
- Add Fehling’s solution;
- Warm;
- Result with Fehling’s solution: Red precipitate
Reagent to confirm the absence of ethanoic acid:
- Add sodium hydrogencarbonate;
- Result: no effervescence observed, hence no acid present;
What is meant by the term hydration?
Addition of water
Why is high-resolution mass spectrometry useful?
useful for identifying compounds that appear to have the same Mr when they’re rounded to the nearest whole number
it is extremely accurate as it measures the relative atomic mass to several decimal places
How does infrared spectroscopy work?
a beam of IR radiation is passed through a sample of chemical
Ir radiation is absorbed by the covalent bond in the molecules, increasing the vibrational energy
bonds between different atoms to absorb different frequencies of IR radiation. Bonds in different places in a molecule absorbs different frequency too.
What is the fingerprint region? Why does it have many peaks?
The area of an infrared spectrum below about 1500cm^-1
the many peaks are caused by the complex vibrations of the whole molecule, which unique for any particular substance
How can you find out if a substance is pure using infrared spectroscopy?
impurities cause extra peaks
Describe how infrared radiation absorption is linked to global warming
Some electromagnetic radiation emitted by Sun reaches Earth and is absorbed
- Earth re-emits some of its infrared radiation (heat)
Molecules of greenhouse gases (CO, CH, water vapour) in atmosphere absorb IR radiation (it’s bonds of these molcules that absorb it)& re-emit some of it back towards Earth = keeps us warm (greenhouse effect)
Means more heat being trapped and Earth getting warmer = global warming
This energy is transferred to other molecules in the atmosphere by collisions (so all atmosphere is warmed)
What causes an increase in greenhouse gas concentrations?
Human activities, burning fossil fuels & leaving rubbish to rot in landfill sites, caused rise in greenhouse gas concentrations
How do you test for alkenes? Draw the equation
Add bromine water, shake, orange to colourless
How do you test for halogenalkanes? Draw the equation
Place test tube in a hot water bath and add NaOH
acidify with HNO3
add AgNO3 (aq)
Precpitate of Ag X will form ( white, cream or yellow)
Describe the test for primary/secondary alcohols
- Add acidified potassium dichromate/ potassium magnate solution
- Warm mixture gently in hot water bath
- Watch for colour change
Orange solution turns green/ purple to colourless
What are the two tests for carboxylic acid. Draw the equations for them
Add sodium/magnesium or sodium carbonate
What is the equation for tollens/Fehling’s
Draw the mechanism for hydration of ethene and write the chemical equation
CH2CH2 + H2O→ CH3CH2OH
The infrared spectrum shown an absorption/ peak in the range 3230 to 3550, which supports the idea that an alcohol is present
the fingerprint region would match with a known sample
Why are carboxylic acids more reactive than alcohols, aldehydes and ketones?
- more reactive than aldehydes and ketones due to containing hydrogen bonding
- O−H bond in COOH is more strongly polarised due to the presence of electron withdrawing carboxy group (C=O) in adjacent position = stronger hydrogen bonds than alchols
