Functional group chemistry Flashcards
List the functional groups in increasing oxidation levels
- The alkane oxidation level
- The alcohol oxidation level
- The aldehyde oxidation level
- The carboxylic oxidation level
What are alkanes known as and why?
Paraffins due to unreactivity
State three properties of alkanes.
- Poorly reactive with ionic or polar substances
- Inert to acids and bases
- Virtually insoluble in water
How are alkanes isolated?
From petroleum through distillation
Describe the melting point of alkanes.
Boiling points and melting points increase with the size of the alkane
Describe the forces between molecules in alkanes
Weak - temporary dipoles , dispersion
What are molecules coated with?
A layer of electrons which occupy bonding and maybe non-bonding orbitals - as a result they are negatively charged (molecules repel each other)
When do reactions occur?
If a pair of molecules have enough energy to overcome the superficial repulsion
Define activation energy
The minimum energy required to react
What brings molecules together?
Charge attraction or orbital overlap
What is the more common cause of organic reaction?
The attraction between a charged reagent ( a cation or anion) and an organic compound that both possess a dipole.
Define electrophile.
Electron acceptors
Define nucleophile
Electron donor
How do we identify nucleophiles?
- They are either negatively charged or neutral species with a pair of electrons in a high energy orbital.
What is the most common type of nucleophile?
Has a non-bonding lone pair of electrons
How do we identify electrophiles?
- They are neutral or positively charged species with an empty atomic orbital or a low-energy anti-bonding orbital that can easily accept electrons
Describe SN1 and SN2 reactions
- Both can predominate
- Use kinetic evidence to explain the mechanisms
What does the rate of an Sn2 reaction depend on?
- The nucleophile
- The carbon skeleton
- The leaving group
- Temperature and solvent
What does the rate of the SN1 reaction depend on?
- The carbon skeleton
- The leaving group
- Temperature
- Solvent
but NOT the nucleophile
Describe the contrasts between SN1 and SN2
- Steric hindrance - drives SN1 reactions as stabilises the carbocation + hinders approach of nucleophile
- Stereochemistry - SN1 reactions loose stereo control + obtain racemic mixture (if starting with a chiral centre) whereas SN2 reactions you invert the stereochemistry
- Solvent effects - SN1 performed in polar protic solvents whereas SN2 performed in aprotic less polar solvents
- Leaving group effect - play an important role in both SN1 and SN2 reactions and halides are driven by pKa and bond strength
- Nucleophiles - Sn1 reactions the nucleophile isn’t important whereas SN2 they are essential
- Amines - give multiple products
- Sulphur is better than oxygen - thiols are more acidic than water - easier to deprotonate
For alkenes what is reactivity controlled by?
- How electron rich the double bond - it is a nucleophile
Describe the double bond in alkenes
- Slightly polar
- pi bond is polarisable, so instantaneous dipole-dipole interactions occur
Describe the conversion of alkenes (isomers)
- Requires breaking of the pi bond between the two sp2 carbons
- Alkene cis to trans conversion
Describe how alkenes can be prepared.
- Cracking - industrial process
- Elimination reactions of halides/alcohols
Describe the mechanism for alkenes.
- Addition reactions via electrophilic addition
- Electron movement from double bond to electrophilic centre
- Addition reactions can either be: homolytic (involves radicals) or heterolytic (involves cations)
- Nucleophile adds to the carbocation/radical
Alkenes - Addition of H-X
What step is carbonation formation?
The rate limiting step
Alkenes - Addition of H-X
What controls the outcome of reaction?
Carbocation stability (tertiary more stable than primary/secondary)
Describe the anti-markovnikov addition (Alkenes)
- In the presence of peroxides HBr adds to an alkene; through a radical mechanism
Why is HBr only involved in the anti-markovnikov addition reaction?
- ONLY HBr has the right bond energy
- HCl bond is too strong, HI tends to break heterolytically to form ions.
Describe the hydrogenation of alkenes to alkanes.
- Syn or Cis addition
- Reaction is stereospecific
- Catalyst required (Pt, Pd or Ni)
Describe Alkynes.
- Unsaturated
- Contains triple C-C bonds; comprised of a sigma bond and two weaker pi bonds
- Reactivity dominated by reactive electron rich C-C triple bond
- Triple bond acts as a nucleophile
- Reactions similar to alkenes
What is the general formula of alkynes?
CnH2n-2
Describe the properties of alkynes.
- Virtually insoluble in water
- Stronger van der Waals forces
- Internal alkynes higher boiling points than terminal
Describe the reactivity of alkynes
- Completely burns in O2
State the reactions alkynes are involved in.
- H-X addition
- X-X addition (Br2, Cl2)
- H-H addition (hydrogenation)
- H-OH addition (hydration)
State the general properties of haloalkanes.
- Act as electrophiles
- Reactivity controlled by electron poor carbon
Describe the general properties of alcohols
- oxygen is sp3 hybridised
- Acts as nucleophiles
- Reactivity controlled by electron rich oxygen
Describe the physical properties of alcohols
- As homologous series increases there is a similar increase in melting and boiling point
- Unusually high boiling points due to hydrogen bonding between molecules
- Small alcohols are miscible in water but solubility decreases as the size of the alkyl group increases
Describe the inductive effect in alcohols.
Acidity decreases as alkyl group increases
What is meant by electron withdrawal (alcohols)?
Halogens increase the acidity
Describe the influence of hydrogen bonding in phenol.
- Raises its melting point and boiling point
- Increases solubility in water
- Higher acidity than alcohol
- Phenoxide stabilised by resonance
Describe the biological importance of epinephrine (phenols).
The principle hormone governing the “fight or flight” response
- Also triggers a variety of physiological events, including increased heart rate
Describe the structure and bonding of amines.
- N is sp3 hybridised with the lone pair of elections in an sp3 orbital
- tetrahedral geometry
- High electrostatic potential at nitrogen; reactivity of nitrogen lone pair dominates properties of amines
Describe the reactivity of amines.
- The lone pair of electrons on nitrogen makes amines basic and nucleophilic
- React with acids to form acid-base salts and they react with electrophiles
Why are amine drugs given in salt form?
- Amine salts are more soluble in water than the parent amines - they will readily dissolve in body fluids
Describe the bonding in carbonyls.
- Trigonal planar sp2 carbon and oxygen
- All sigma bonds lie in the same plane; 120 degrees apart
- C-O pi bond between parallel p orbitals on carbon and oxygen
Describe carbonyl interactions at binding sites.
- Planar, significant dipole moment
- May interact with binding site through:
Hydrogen bonding (carbonyl group is a hydrogen bond acceptor and two lone pairs on oxygen) - dipole-dipole interactions
Describe reactions at the carbonyl carbon
- Electron pair moves from C=O bond to electronegative oxygen atom producing tetrahedral alkoxide ion intermediate
- Formation of new bonds increases steric crowding
- Introduction of a chiral centre (carbonyl carbon sp2 -> sp3)
State good nucleophiles for the reactions at the carbonyl carbon
Hydride
Alkynyl anion
Alkoxides
How can the reactivity of carbonyl groups be enhanced?
By protonation of the carbonyl oxygen (to make the conjugate acid)
