Organic Flashcards
Why does Br2 react more readily with alkenes than benzene?
-pi bond in alkane has localised e‐ above and below plane of the C=C
- increases e- density and induces dipole in Br2 to act as electrophile
- benzene has delocalise e- above and below C ring and e- density is less that C=C
- not enough density to polarise the bromine to act as an electrophile
What is benzene’s bond angle and the bond lengths compared to a single and double covalent bond?
-benzene is planar with bond abgle of 120⁰
-all bond lengths are equal
- bond lengths are smaller than single C-C bonds but longer than C=C bonds
What are the reagents for reducing nitrobenzene to aminobenzene?
- Sn and conc. HCl under reflux forms a phenyl ammonium ion
-add NaOH
What forms when an acyl chloride reacts with water?
- a carboxylic acid + HCl
- vigorous reaction and white misty fumes of HCl gas produced
What does the Bromination of benzene require?
A halogen carrier (AlBr3)
Why does cyclohexene not require a halogen carrier when undergoing bromination?
-in alkenes the pi electrons are concentrated between the two carbons
- electronic density is high enough for electrophiles to be attracted to alkenes
Describe the formation of 2-nitrophenol
Phenol + dilute nitric acid at RTP
Why does phenol act as a weak acid?
-When phenol dissociates, a phenoxide ion forms and H+ forms
- due to the high electronegativity of the oxygen on the phenoxide, demoralised electrons are drawn towards it
- causes a high charge around the oxygen and attracts H+ back again
- equilibrium is to the LEFT
How do you reduce aldehydes and ketones to alcohols
Aqueous Sodium borahydride (NaBH4) and H+
Why might you expect compounds with similar Mr values to have similar boiling points?
They will have a similar amount of London forces between molecules
What is the colour change when KMnO4 is added to an aldehyde?
Purple to colourless
Why is HCN not used to provide the CN- nucleophile?
- HCN is a highly toxic gas and difficult to store
-KCN is a white solid= dissolve in water with small vol. of sulfuric acid to form CN-
In terms of rates of reaction, why is KCN a better reagent than HCN when lengthening a carbon chain?
- HCN is a weak acid so does not fully dissociate into H+ and CN- ions
- rate pf reaction is slower with HCN as KCN fully dissociates
Why does optical isomerism occur in the products formed when reacting aldehydes with CN-?
- the C=O bond is planar so the CN- can attack from either side
- this causes the optical isomers
What is needed in excess to form a secondary amine and why?
- excess primary amine with RX
- ensures there is sufficient primary amine to attack the remaining haloalkanes
-ensures no further substitution takes place
Why can amines act as Bronsted Lowry Bases?
- there is a lone pair on the N atom
- it can accept a proton
- forms a dative bond
Why are polyesters more reactive than addition polymers?
-Molecules in addition polymers are fully saturated so they are non-polar (the molecules are chemically inert)
-Polyesters contain polar bonds so are able to react with more (acids/ alkalis/ nucleophiles)
How does the reactivity of polymers affect their biodegradability?
-polyalkenes are non-biodegradable because they are fully saturated / they are chemically inert
-polyamides and polyesters have polar bonds so are able to react with nucleophiles etc
Define molecular formula
The actual number of atoms in a molecule or element
Define structural formula
arrangement of atoms in a molecule without showing all the bonds
Define empirical formula
Simplest whole number ratio of atoms in a compound
Define skeletal formula
Show the bonds of the carbon skeleton only (H and C atoms not shown but functional groups are)
Define displayed formula
Show the arrangement of atoms showing all bonds and atoms in the molecule
Define a homologous series
- a group of compounds that contain the same functional groups and have the same general formula
- successive members in same homologous series differe by CH2
What are aromatic compounds?
Compounds that contain a benzene ring which may have different functional/alkyl groups coming from this
What are alicyclic compounds?
Rings that aren’t aromatic (have same general formula as alkenes)
What are aliphatic compounds?
Straight, branched or non aromatic chains
What are the 2 types of bond fission?
- heterolytic fission= bond breaks and electrons are distributed unequally to form 2 different ions double headed arrow shows movement of a pair of electrons
- homolytic fission= bond breaks and pair of electrons shared equally to form 2 uncharged radicals
dot shows unpaired electron on atom
Define structural isomerism
the isomers have same molecular formula but different structural formulae
What are the 3 types of structural isomerism?
- chain isomerism= same molecular formula but different arangement of carbon skeleton
- positional isomerism= same molecular formula but different position of functional group on carbon skeleton
- functional group isomerism= same molecular formula but different fuctional group
Describe the shape of alkane molecules
- always from tetrahedral shape around carbon atom (angle around each bond is 109.5)
- bonds repel eachother equally as bonds contain electrons so will want to be as far apart as possible
How does branching and length of chain affect bp?
- no branching means molecules can pack together closely = more surface contact = higher bp ac induced dipole-dipoles between molecules are stronger
- longer, straight chained hydrocarbons have more London forces so more energy needed to over come forces= bp increases
- branched hydrocarbons can’t pack together as closely which weakens London forces
Bigger molecules or atoms have more London forces due to larger electron clouds
Describe free radical substitution reactions
- Initiation= radicals produced by visible light/UV light and bond breaks via homolytic fission (2 radicals)
- Propogation= radical reacts with non-radical and new radicals are created which can also react with non-radical molecules chain reaction
- Termination= when 2 radicals react and form a non-radical molecule which ends chain reaction
Describe the chlorination of methane to form chloromethane
- Initiation= sunlight (UV) breaks Cl-Cl bond by photodissociation and bond breakes homolytically and produces 2 highly reactive Cl*
- Propogation= Cl* reacts with methane to form CH3* + HCl
- CH3* then reacts with Cl2 to form chloromethane and Cl*
- the Cl* can react with more methane - Termination= Cl* can react with CH3* to form chloromethane and other radicals react with eachother etc.
Why is it difficult to produce just chloromethane during free radical substitution?
- excess Chlorine:
- we will get di, tri and tetra haloalkane which all ahve to be separated - excess methane:
- we will reduce amount of multiple substitutions as there is a greater chance for chlorine reacting with methane
- isomers also form because free radical substituion can occur anywhere on a hydrocarbon
Describe sigma bonds
- sigma bonds form when 2 s orbitals overlap and align horizontally to give a siongle convalent bond
- strong electrostatic attraction between nuclei and shared pair of electrons due to high electron density between nuclei= high bond enthalpy and strongest type of bond
Describe pi bonds
- parallel overlap of 2 p-orbitals which have a dumbell shape= when they merge they form oblong shapes (one above and below bond)
- pi bonds weaker than sigma bonds as electron density spread out above and below nuclei (lower bond enthalpy due to decreased electrostatic attraction)
What does a double bond contain and why does it make alkenes so reactive?
- a sigma and pi bond
- high electron density as four electrons are shared in double bond (very reactive)
- open to attack from electrophiles = pi bond sticks out and whole double bond has high electron density
- pi bond has low bond enthalpy so alkenes are reactive
Define steroisomerism
- same structural formula but different arrangement of atoms in space
- e.g E/Z isomerism
Define electrophiles and nucleophiles
- electrophile= electron pair acceptor as they are electron deficient
- nucleophile= electron donator as they are negatively charged species
Why are alkanes not as reactive as alkenes?
- alkanes only have sigma bonds and they are not polar so are not as reactive as alkenes which have both sigma and pi bonds
What determines the amount of products that will form from unsymmetrical alkenes when reacting with HX?
- determined by stability of carbocation intermediate
- more alkyl groups bonded to carbocation, the more stable it is
- this is because alkyl groups push electrons twoard positive carbocation and stabilise it
PRIMARY= 1 R group
SECONDARY= 2 R groups
TERTIARY=3 R groups (major)
Describe Markownikoff’s rule
- the major prodcut when we add a hydrogen halide to an unsymmetrical alkene is where the H adds to carbon with most number of hydrogens already
What are the benefits and disadvantages of landfills to dispose of poymers (plastics)?
:)
- useful for plastics that are difficult to recycle
- too difficult to separate from other materials
:(
- landfill is not very sustainable as large amounts of land is needed (deforestation)
- increasingly expensive to use land for waste disposal