1.7 Aldehydes and Ketones Flashcards
what functional group do aldehydes and ketones have?
carbonyl group (C=O)
Name some uses of aldehydes and ketones
solvents (eg. propanone/acetone, MIBK) plastic manufacture (formaldehyde) flavour of fruits (eg. ethanal smells like apples)
What is the structural difference between aldehydes and ketones?
aldehydes have the carbonyl group on the end carbon (ie carbon with the carbonyl group will have at least one H bonded to it)
Ketones will have the carbonyl group bonded to any group other than the end carbons (ie there will be 2 C atoms bonded to the carbonyl group)
What is the shortest ketone?
propanone
what is the shortest aldehyde?
methanal
what is the name of an aldehyde with a C=C?
enal
what is the name of an aldehyde with 2 C=C s?
dienal
what is the name of a benzene ring with a CHO attached?
benzaldehyde
What is the name of a ketone with a C=C?
enone
what is the name of a ketone with 2 C=C s?
dienone
how is a ketone referred to when in a molecule containing functional groups with higher priority than carbonyl?
keto- prefix
what does the C=O bond consist of?
σ-bond and a π-bond
describe the polarisation in C=O
O is more electronegative, therefore pulls the electrons in the bond towards it, resulting in greater electron denser over the more electronegative oxygen. As a result, C=O is polarised.
describe the boiling point of an aldehyde and ketone
- Boiling points rise as molecules get bigger - due to VdW (longer carbon chain means more electrons and increases strength of instantaneous dipoles)
- Methanal is a gas, and ethanal boils at room temperature. All other aldehydes and ketones are liquids.
- Boiling point will be higher than that of similar sized hydrocarbon (due to dipole-dipole interactions), but lower than alcohols (lack of H bonding)
describe the bonding between molecules in an aldehyde or ketone
- dipole-dipole interactions (due to polarity of C=O)
- van der waal’s
- NO hydrogen bonding (they don’t have any hydrogen atoms attached directly to the oxygen, and therefore can’t H bond with each other. they can H bond with water however)
Describe the solubility of an aldehyde or ketone in water
- Small aldehyde and ketones are freely soluble in water, but solubility falls with chain length.
- As chain lengths increase, the hydrocarbon ‘tails’ get in the way, and force themselves between water molecules, breaking the H bonds
- Although aldehydes and ketones can’t H bond with themselves, they can H bond with water molecules.
- One of the slightly positive H atoms in water will be sufficiently attracted to the lone pairs on oxygen of an aldehyde that a H bond will be formed
describe the similarities and differences between the C=C and C=O bond
- C=C has high electron density at double bond, C=O has electron deficient carbon
- both have pi and sigma bonding
- C=C is non-polar, C=O is polar
- C=C undergoes electrophilic addition, C=O undergoes nucleophilic addition
what is the result of oxidising ethanol (a primary alcohol)?
ethanal and water
what is the result of oxidising ethanal?
ethanoic acid
define primary alcohol
alcohol where the -OH group is attached to a carbon that is attached to one other carbon (ie also attached to two hydrogen atoms)
what atoms are removed from a primary alcohol when it is oxidised?
- one of the H atoms and the -OH group
- another hydrogen can be removed to be oxidised further to carboxylic acies
describe what can happen to methanol when it is oxidised
- first oxidation -> methanal
- second oxidation -> methanoic acid
- third oxidation -> third hydrogen is removed and hydroxymethanoic acid is formed.
describe the practical theory surrounding oxidation of a primary alcohol
- If aldehyde is the required product, distillation apparatus must be used instead of reflux - prevention from further oxidation to carboxylic acid
- Aldehydes have a lower boiling point than alcohols
- If carboxylic acid is required product, can be performed under reflux to allow ethanol to undergo full oxidation
- Oxidation of ethanol to ethanal will turn from a spirit smell to a sweet, apple smell
name an oxidising agent
acidified potassium dichromate (K+/K2Cr2O7)
acidified potassium manganate (VII) (H+/MnO4)
give the equation for the colour change of acidified potassium dichromate during oxidation
Cr2O7^2- + 14H^+ + 6e^- -> 2Cr^3+ + 7H2O
give the colour change during oxidation with acidified potassium dichromate
orange Cr^6+ is reduced to green Cr^3+
give the equation for oxidation with acidified potassium manganate (VII)
MnO4^- + 8H^+ + 5e^- -> Mn^2+ + 4H2O
give the colour change during oxidation with acidified potassium manganate (VII)
purple Mn^3+ reduced to colourless Mn^2+
define secondary alcohol
alcohol where the carbon bearing the -OH group is attached to two other carbon atoms (therefore one hydrogen)
how many oxidations can occur with a primary alcohol?
2 normally, 3 with methanol
how many oxidations can occur with a secondary alcohol?
just one - there are no further H atoms bonded to the carbonyl carbon
describe the oxidation of tertiary alcohols
Tertiary alcohols can’t be easily oxidised as there are no H atoms bonded to the carbon bonded to the -OH group
describe the reduction abilities of aldehydes and ketones
- Aldehydes have reducing properties - they can reduce other substances while they themselves are oxidised
- Ketones can’t act as reductants as they can’t be oxidised any further
name some reagents you can use to distinguish between aldehydes and ketones
acidified potassium dichromate
tollen’s reagent
fehling’s solution
describe how to use acidified potassium dichromate to distinguish between aldehydes and ketones
Add acidified potassium dichromate to samples in boiling tubes and heat in a water bath
Aldehydes:
Colour change = orange -> green
Change in smell = sweet -> acidic
Ketones:
Remains orange
state the other name for tollen’s reagent, the formula and the active agent
ammoniacal silver nitrate [Ag(NH3)2]NO3 - active agent viewed as Ag+
describe how to form tollen’s reagent
- Small quantity of aqueous sodium hydroxide added to aqueous silver nitrate, forming ppt of hydrated silver oxide Ag2O
- Addition of aqueous ammonia causes ppt to form a colourless solution containing the silver amine complex
describe the tollen’s reagent test to distinguish between aldehydes and ketones
- On warming with an aldehyde the colourless reagent is reduced to a grey ppt of metallic silver. In a thoroughly clean and dry glass container it forms a silver mirror
- Ag+(aq) + e- -> Ag(s)
- Ketones do not react with Tollen’s reagent
describe what Fehling’s solution is
a blue solution containing a complex of Cu^2+(aq)
describe how to form Fehling’s solution
Mix Fehling’s A (aqueous copper (II) sulphate) with Fehling’s B (alkaline solution of sodium potassium tartrate
describe the Fehling’s solution test to distinguish between aldehydes and ketones
On warming, aldehydes reduce Fehling’s which is blue, to a brick red precipitates of copper (I) oxide Cu2O
Ketones do not reduce Fehling’s solution (remains blue)
describe the results of performing the Fehling’s solution test on methanal
Methanal will reduce Cu2O even further to metallic copper because methanal can lose another H
describe the difference between Benedict’s solution and Fehling’s solution
Benedict’s uses citrate ions instead of tartrate ions. Aldehydes are reduced and ketones are not.
Give the name for reducing sugars and give an example
Reducing sugars eg. Glucose are called aldose sugars - contain aldehyde functionality
Give the name for non-reducing sugars and give an example
Non-reducing sugars eg fructose are called ketose sugars - contain ketone functionality
name a reagent that can reduce aldehydes and ketones
lithium aluminium hydride (lithal/lithium tetrahydridoaluminate/LiAlH4) or sodium borohydride/tetrahydrido borate NaBH4
what are aldehydes and ketones reduced to?
their corresponding 1º and 2º alcohols
how do reducing agents work?
they are hydride carriers as they contain the hydride ion H- which acts as a nucleophile and attacks the C=O bond in nucleophilic addition reaction, reducing aldehydes to primary alcohols and ketones to secondary alcohols
describe the conditions necessary to use lithal
Lithal is a powerful reductant and will react vigorously and violently with water if it is present in the reaction mixture. Therefore lethal must only be used in anhydrous conditions. Dry ether (CH3OCH3) is the solvent most commonly used. Water is added at the end of the reaction to complete the final step in the mechanism.
why don’t lithal and NaBH4 reduce C=C bonds?
they will target the slightly positive carbon in C=O, but C=C doesn’t have a permanent dipole
name an indiscriminate reducing agent that is used with ketones and aldehydes
the C=C group will be reduced if the aldehyde or ketone is passed over hydrogen in the presence of a nickel or platinum catalyst - again forming the primary or secondary alcohol from which it was derived.
describe why nucleophiles will attack the C=O bond
The δ+C of the C=O bond is election poor and therefore is attacked by nucleophiles resulting in nucleophilic addition reactions
what is the relation between the rate of nucleophilic addition and the positive charge?
- The rate of nucleophilic addition to the carbonyl group increases with the size of the positive charge on the carbon atom
- Groups that ‘push’ electrons towards the δ+C of the C=O group will decrease the value of the charge.
- The order of reactivity is therefore: methanal > aldehyde > ketone
Describe how to define the strength of a nucleophile
- The strength of a nucleophile is the extent to which a species can donate a pair of electrons
- An anion with a full negative charge will be a stronger nucleophile than a partial negative charge
list common nucleophiles in order of strength
:CN- > :OH- > :NH3 >H2O >H3O+
describe how HCN is collected to be reacted with aldehydes and ketones
HCN is a toxic gas and is therefore produced in situ by the reaction of NaCN/KCN with sulfuric acid.
describe the conditions required to react an aldehyde or ketone with HCN
room temperature
HCN made in situ
How can the H+ concentration be increased when reacting HCN with aldehydes or ketones?
HCN -> H2O)
- add a few drops of mineral acid eg sulfuric acid, which creates a supply of H+ ions (but not from the HCN)
Describe the mechanism when HCN is reacted with an aldehyde
- HCN dissociates into H+ and CN-
- nucelophilic :CN- attacks delta +ve carbon in C=O. Electrons move from :CN- to carbon, then from carbon to oxygen
- resulting in an intermediate molecule of a carbon bonded to a H, R group, CN, and O:-
- electrons from O:- move to H+, causing the group to become -OH
name the species produced after the reaction between an aldehyde and HCN
hydroxynitriles
it also increases the chain length in comparison to the original aldehyde due to the addition of the -CN group
Describe how to identify a compound by derivative formation
- make a derivative of the unknown compound
- determine the melting point of the derivative
- use the melting point to identify the derivative
- identify the compound used to make the derivative
describe how to identify a compound by instrumental analysis
- use the mass spectrum of the unknown compound to deduce the molecular mass and the molecular fragments
- use the IR spectrum of the unknown to deduce the specific bonds present and the functional groups present
- determine the structure of the unknown compound
Describe the structure of 2,4-dinitrophenylhydrazine
:NH2-:NH- attached to a benzene ring with two nitro (NO2) groups attaches at carbon 2 and 4
describe the mechanism of reaction of a ketone with 2,4-DNP
- electrons in :NH2 move to electronegative carbon in C=O of ketone
- electrons move from carbon to oxygen
- 2,4-DNP attaches to ketone in place of oxygen
- hydrogens from NH2 and oxygen from ketone form water (condensation reaction)
describe how the reaction between 2,4-DNP and aldehydes/ketones is used for identification purposes
- Each aldehyde or ketone will produce a different 2,4-dinitrophenylhydrozone derivative, which will have a precise melting point
- The melting point of the derivative can be compared with tables to allow determination of the aldehyde or ketone that was initially present.
give a method for a practical to identify an unknown aldehyde or ketone
- Place 5 cm³ of 2,4-dinitrophenylhydrazine solution in a suitable container.
- Add some drops of the test liquid e.g. propanal or the test solid dissolved in ethanol.
- If crystals do not form add some dilute sulfuric acid and warm the mixture.
- Cool the mixture in iced water.
- Filter off the crystals using suction filtration.
- Recrystallise - Dissolve the impure crystals in the minimum volume of hot solvent. Filter when hot by gravity filtration, using a hot funnel, or fluted filter paper, to remove insoluble impurities. Allow filtrate to cool and crystallise
- Filter off the crystals using suction filtration
- Dry by sucking air over the crystals in the Buchner and then in a low temperature oven/dessicator
- Determine the melting point
describe the suction filtration method
- Place filter paper in a Buchner funnel
- Place Buchner funnel in a Buchner flask
- Attach the flask to a suction pump and suck air through the flask
describe the melting point method
- Place some solid in a capillary tube/melting point tube sealed at one end
- Heat slowly (using melting point apparatus)
- Record the temperature at which the solid starts and finishes melting
- Repeat and average the temperatures
- Compare the temperatures with known values in a data book
