6-Further Organic Chemistry Flashcards
How are aldehydes produced?
Oxidation of primary alcohols
What is a ketone?
Two hydrocarbon groups attached and a carbonyl group
How are ketones produced?
Oxidation of secondary alcohols
How does the solubility of aldehydes and ketones change?
Decreases with increasing chain length
As chain length increases, the hydrocarbon portions of the molecules prevent such attractions occurring
Why are aldehydes and ketones soluble in water?
Form hydrogen bonds with water molecules
Dispersion forces and dipole-dipole attractions between it and the water molecule exist. This releases energy which supplies energy needed to separate water molecules
Draw a diagram for how aldehydes and ketones form hydrogen bonds with water
Sheet
What is the carbonyl group?
C=O
What is Brady’s reagent?
2,4-dinitrophenylhydrazine
Red orange solid, usually supplied wet to reduce the risk of explosion
What is the displayed formula of Brady’s reagent?
1.6.24
What do aldehydes and ketones react with Brady’s reagent to form? What are they called?
Yellow/orange/red crystalline solids
2,4-dinitrophenylhydrazine derivatives
What is Brady’s reagent used for?
Identifying aldehydes and ketones
What sort of reaction occurs between Brady’s reagent and aldehydes or ketones?
Condensation
Addition followed by elimination of water
How is the 2,4-dinitrophenylhydrazine derivative removed?
Filtration and purified by recrystallisation fig 1.6.27
Why use Brady’s reagent?
The derivatives are less soluble and therefore crystallise out more easily, so it is more useful
What are aldehydes readily oxidised to form?
Carboxylic acids
What are ketones oxidised to form?
Don’t oxidise readily
How can the oxidisation of aldehydes be carried out?
Warming with a solution of potassium dichromate (VI) acidified with sulphuric acid
What are the most common tests to determine whether it is an aldehyde or ketone?
Fehling’s solution
Tollen’s reagent
Acidified dichromate (VI) ions
Benedict’s solution
What is Fehling’s solution?
A - Cu2+ complexes with sodium dihydroxybutanedioate
B- NaOH
What is Benedict’s solution?
Alkaline solution Cu2+ ions
Cirate ions
What happens when Fehling’s or Benedict’s solution is used on an aldehyde or ketone?
Aldehyde
Red-brown precipitate of copper (I) oxide
Ketone
No reaction- stays blue
What the equation for the reaction of aldehydes with Fehling’s solution?
2Cu2+ + OH- + RCHO + H2O –> Cu2O(s) +RCOOH + 3H+
What is an aldehyde?
C=O bond with a H attached to the same C atom
What the equation for the reaction of aldehydes with Tollen’s reagent?
2Ag+ + RCHO +H2O –> 2Ag(s) +RCOOH + 2H+
No reaction with ketone
What are aldehydes and ketones used to reduce? What is the reagent required?
Aldehydes - primary alcohols
Ketones - secondary alcohols
Sodium tetrahydridoborate (NaBH4) dissolved in ethanol
How are hydroxynitriles formed?
Aldehydes and ketones undergo addition reactions with HCN in presence of a KCN catalyst
The nucleophile, CN-, attacks the central carbon atom, displacing a pair of electrons in the double bond and pushing them onto the oxygen atom, leaving the oxygen atom with negative charge
The oxygen then acts as a base and gains a proton from the HCN, reproducing the CN-
Forms a racemic mixture
What is formed in the triiodomethane reaction and what reagents are used?
CHI3 and sodium alkanoate
Excess NaOH and iodine
What is triiodomethane?
Insoluble so forms a yellow crystalline precipitate
What gives a positive result for the triiodomethane reaction?
Contain a methyl group next to a carbonyl group or can react to give a methyl group next to a carbonyl group
What is the equation for the triiodomethane reaction?
Look up
What are the properties of carboxylic acids governed by?
Polarity of the carboxyl group and the length of the carbon change
As the length of the chain increases, the influence of the functional group decreases
How does hydrogen bonding affect carboxylic acids?
Relatively high melting and boiling temperatures
Soluble in water. Longer chains become less soluble because the organic nature of the carbon chain becomes dominant
More than 8 carbon atoms - solid at room temperature and slightly soluble in cold water but more so in hot
How can carboxylic acids be prepared?
Prolonged oxidation of primary alcohols or aldehydes, for example heating with acidified potassium dichromate (VI) under reflux
Hydrolysis of a nitrile by boiling with dilute acid
RCN + 2H2O + H+ –> RCOOH + NH4
How can carboxylic acids be reduced?
Only possible to reduce back to alcohol
Lithium tetrahydridoaluminate suspended in a dry ether at room temperature
Addition of water at end with destroy any excess LIAl4
RCOOH + 2[H] –> RCH2OH + H2O
What happens when a carboxylic acid is dissolved in water?
Dissociates to form H+ and carboxylate ions
Behaves as an acid
Further anaylsis shows CO bonds identical (delocalised charge) so doesn’t attract cations as strongly
What is the equation of a carboxylic acid with a base?
RCOOH + NaOH RCOO-,Na+(s) + H2O
What are the properties of the salts formed from the reaction between carboxylic acids and water?
Ionic so have crystalline structures and readily soluble
Polar carboxylate group is hydrophilic and carbon chain is hydrophobic which mix with grease while the other parts stay outside
Negatively charged polar heads repel each other, allowing them to be carried away by water
How does the length of a carbon chain affect the solubility of the salts formed from the reaction between carboxylic acids and water?
Short chains show mainly ionic character
Long chains show mainly organic character
Medium chains have a combination of the characteristics and are used in soaps and detergents
What reacts with phosphorus (V) chloride? What are the products?
Test for OH group (carboxylic acid and alcohol)
POCl3 + HCl + Cl substituted for OH
How is an ester formed?
Carboxylic acid mixed with alcohol in presence of strong acid catalyst such as HCl
Esterification
Slow equilibrium reaction so more reactive chemicals often used such as (ethanoyl) chlorides or (ethanoic) anhydride
How is an ester named?
Name starts with ‘alyl word’ from alcohol used
Followed by acid ending with ‘-oate’
What are the properties of esters?
Volatile
Relatively low melting and boiling temperatures
Not very soluble in water or polar substances
Distinctive fruity smells
What are esters used for?
Manufacture of soap and detergents
Solvent in the manufacture of adhesives
What reagents and conditions are needed for acid hydrolysis of an ester? Give an equation
Boiling with water and using dilute acid
Dilute acid provides hydrogen ions which catalyse reaction
CH3COOC2H5 + H2O –> CH3COOH + CH3CH2OH
What reagents and conditions are needed for base hydrolysis of an ester? Give an equation
Boiled with potassium hydroxide solution
More efficient process because when acid is released it is neutralised by alkali to form potassium salt (doesn’t react with alcohol)
CH3COOC2H5 + OH- –> CH3COO- + CH3CH2OH
How is a polyester formed? What is produced? What type of reaction is it?
Dicarboxylic acid and diol react
Water produced and OH bonds break to form ester linkage
Condensation polymerisation
Diacyl chlorides and diacid anhydrides can be used in place of dicarboxylic acid
How are acyl chlorides produced? How do they react in comparison to carboxylic acids?
PCl5 react with carboxylic acid
React in same way but more readily because Cl causes delta + charge on C so more susceptible to attack by nucleophiles
How do acyl chlorides react with water?
Hydrolysed rapidly with water (even from air)
CH3COCl + H2O –> CH3COOH +HCl
How do acyl chlorides react with alcohols?
Produce esters and HCl
Mixing without heat
What is the mechanism for the reaction between acyl chlorides and alcohols?
Lone pair on O from OH attracted to delta + C in acyl chloride
Breaks bond with Cl to produce Cl-
O has positive charge
H from OH bonds with Cl- to form HCl
How do acyl chlorides react with concentrated ammonia?
Form amides without heating
CH3COCl + NH3 –> CH3CONH2 + HCl
What does ‘n-methyl’ mean?
Methyl group is attached to nitrogen atom?
How do acyl chlorides react with amines?
N-methylethanamide
CH3COCl + CH3NH2 –>CH3CONHCH3 + HCl
What is optical isomerism?
Form of stereoisomerism
Mirror images of each other/ not superimposable
Chiral molecules with chiral centre indicated by *
What are enantiomers?
Different forms of optical isomers
Similar chemical and physical properties but are optically active (different effects of plane-polarised light)
Biological systems are sensitive to difference, eg only D-propoxyphene used as painkiller
What is plane-polarised light?
A wave in which the oscillations occur in one plane only
How does a polarimeter work?
Optically active mixture rotates plane-polarised light
Rotate second piece of polariod to stop all light coming through, called angle of rotation
Source of light is monochromatic (single wave length)
Gives optical activity of substance
What is a racemic mixture?
Enantiomers are in 1:1 ratio
Clockwise rotation counteracts counterclockwise rotation
Not optically active
What is the Sn2 mechanism?
Arrangement is turned inside out in the process called inversion of configuration
Centre temporarily has 5 bonds
Negative ion removed
Rotation can be opposite but sometimes has no effect
What is the Sn1 mechanism?
Bond with electronegative ion breaks
-OH can attack from either above or below
Forms a racemic mixture
Why is the C=O bond permanently polarised?
Oxygen has a much higher electronegativity so electrons are attracted more strongly to oxygen atom, especially in pi bond
