Module 4 Flashcards
Hydrocarbon?
Compounds that contain only H and C
Aliphatic?
A hydrocarbon with carbon atoms joined in straight chains or branched chains
e,g, hexane
Alicyclic?
A hydrocarbon with carbon atoms joined together in a ring structure
e.g. cyclohexane
Aromatic?
A hydrocarbon with at least one benzene ring in the structure
e.g. benzene
Homologous series?
- same functional group
- differ from successive members by a unit of CH2
- properties show a gradation
Alkanes general formula?
CnH2n+2
Alkenes general formula?
CnH2n
Alcohols general formula?
CnH2n+1OH
Prefix?
- 1st part of the name
- other functional groups
Stem?
- main part of the name
- derived from the longest carbon chain
Suffix?
- last part of the name
- most important functional group
Mutiplying Prefixes?
-shows presence of one or more groups
Locants?
-numbers and hyphens used to show positions of groups within molecules
number of carbon atoms prefix?
1=methyl 2=ethyl 3=propyl 4=butyl etc.
1-2-dichloro-pentan-3-ol?
1,2=locant di=multiplying prefix chloro=prefix pentan=stem 3-ol=suffix
Functional Group?
the group of atoms that are responsible for the characteristics of a compound
How is the functional group placed?
- suffix: if most important group
- prefix- if additional functional group
Alcohol:
prefix?
suffix?
prefix=hydroxy-
suffix=-ol
What’s an alkyl group?
if you remove a hydrogen atom from an alkane you get an alkyl group stems: 1C=methyl 2C=ethyl etc.
Aldehyde:
- prefix?
- suffix?
suffix= -al
prefix=
Alkane:
- prefix?
- suffix?
prefix=
suffix= -ane
Alkene:
prefix?
-suffix?
prefix=
suffix= =ene
Carboxylic acid
- prefix?
- suffix?
prefix=
suffix= -oic acid
Haloalkane:
prefix?
suffix?
prefix= fluoro,chloro,bromo,iodo
Ketone:
prefix?
suffix?
prefix=
suffix= -one
What are the 6 different types of formulae?
- general
- skeletal
- display
- empirical
- molecular
- structural
General formula?
- simplest algebraic formula for a homologous series
e. g. methane = CH4
Structural formula?
-provides the minimum detail to show the arrangement of atoms in a molecule
(sticks and h’s incl.)
Skeletal formula?
-simplified structural formula drawn by removing hydrogen atoms from alkyl chains
lines=bonds between atoms
junctions= carbon atoms
Display formula
-shows relative positioning of atoms and bonds between them
Molecular formula?
-shows number and types of atoms of each element in a compound
Empirical formula?
-simplest whole number ratio of each element present in a compound
Isomer?
-compounds with same molecular formula but different arrangement of atoms
Isomer types?
- structural
- stereisomers-e/z isomers
Structural isomer?
- compounds with same molecular formula but different structural formula
- 2 types:
- chain isomerism-molecules with different carbon chains
- position isomers: molecules with the same functional group that is attached in different positions
Stereoisomers?
-organic compounds with the same molecular formula and structural formula but have a different arrangement of atoms in space.
E/Z Isomerism?
- type of stereoisomerism caused by the restriced rotation around the double bond (C=C)
- 2 different groups are attached to both carbon atoms of the c=c bond
How is priority assigned to e/z?
cahn-ingold
- priority assigned to substituents
- based on the atomic mass of the group(higher mass=higher priority)
if priority groups on same side of c=c then the isomer is Z
if priority groups are on different sides of the c=c then the isomer is E
What is Z cis and E trans? isomers
Z=2 priorities same side
E=2 priorities opposite side
how can structural isomers occur?
1- alkyl groups are in different places
2- the functional group can be bonded to different parts of the parent chain
3- the functional group could be different
What is a reaction mechanism?
models that show the movement of electron pairs during a reaction
curly arrows?
model the flow of electron pairs during reaction mechanisms
radicals?
species with 1 or more unpaired electrons
Homolytic fission?
- when a covalent bond breaks and each electron goes to a different bonded atom.
- generates 2 highly reactive neutral species called radicals - shown as dots
- X+Y= X• + Y•
Heterolytic fission?
- when a covalent bond breaks and both electrons go to one of the bonded atoms.
- results in a cation or anion being formed
- X -Y = X + + Y-
covalent bond fission?
during the 1st stage of a chemical reaction, activation energy affects the covalent bonds-causing them to break - undergoing fission
example of homolytic fission?
- e.g in a Cl2 - there’s a shared pair of electrons between the atoms.
- when UV radiation strikes the covalent bond it causes it to break.
- 1 electron from the covalent bond goes to each chlorine atom forming 2 identical radicals:
- Cl-Cl= (UV) 2Cl•
example of heterolytic fission?
- e.g. 2-bromopropane has a molecule that has a dipole
- the Br atom is more electronegative than the carbon atom, so the electrons spend more time near the Br than the C atom.
- this makes the Br atom δ- and the C atom δ+
- under certain conditions, the polar covalent bond will undergo heterolytic fission. both electrons from the bond go to to the Br, making a bromide ion and leaving a carbocation.
How can covalent bonds form with radicals?
- when 2 radicals or 2 oppositely charged ions collide, a new bond is formed.
e. g. in production of tetrachloromethane, methyl radicals are made. when 2 methyl radicals collide, they form ethane.
What are carbocations?
-very reactive intermediates because they’re unstable
Draw a reaction mechanism to show homolytic fission in the reaction between chlorine and methane to form chloromethane under UV light.
drawing
Draw a reaction mechanism to show the tetrabromoethane from 2 radicals
Drawing
Draw a reaction mechanism to show the reaction between a carbocation and an iodide ion to form iodethane.
drawn
What are alkanes?
- hydrocarbons containing H and C only
- saturated (single covalent bonds only)
- general formula CnH2n+2
- have a gradation in physical properties
- have similar chemical properties
Bonding in alkanes?
- C has 4e- in outer shell and pair up with e- on other atoms to form 4 covalent bonds
- each covalent bond has direct overlap of the electron clouds from each atom making a sigma bond.
- therefore c-c and c-eh bonds are examples of σ bonds
What is the shape and bond angle of alkane structure?
-every carbon has a tetrahedral shape with a bond angle of 109.5 because there are 4 bond pairs of electrons equally repelling and 0 lone pairs
How does a σ bond form?
- One sp2 orbital from each carbon overlap to form a single C-C bond called a sigma σ bond
- Rotation can occur around a sigma bond
What’s the boiling point in alkanes?
-increasing boiling points of the alkane homologous series due to the increasing number of electrons in the bigger molecules causing an increase in the size of the induced dipole–dipole interactions (London forces) between molecules.
What else can have an effect on the size of ldn forces (alkanes)
The shape of the molecule can also have an effect on the size of the induced dipole–dipole interactions
(London forces) . Long chain alkanes have a larger surface area of contact between molecules for London
force to form than compared to spherical shaped branched alkanes and so have stronger induced dipole– dipole interactions and higher boiling points.
Describe the reactivity of alkanes
The low reactivity of alkanes with many reagents can be explained by the high bond enthalpies of the C-C
and C-H bonds and the very low polarity of the σ-bonds present.
Give the equation for the complete combustion of alkanes
C8H18(g) + 12.5 O2(g) → 8CO2(g) + 9 H2O(l)
The products of complete combustion are CO2
and H2O.
Alkanes readily burn in the presence of oxygen. This
combustion of alkanes is highly exothermic, explaining their use as fuels.
Give the equation for the incomplete combustion of alkanes
If there is a limited amount of oxygen then incomplete combustion occurs, producing CO (which is very toxic) and/or C (producing a sooty flame)
CH4(g) + 3/2 O2(g) → CO(g) + 2 H2O(l)
CH4(g) + O2(g) → C(s) + 2 H2O(l)
Incomplete combustion produces less energy per mole than complete combustion
Why is soot bad?
Carbon (soot) can cause global dimming- reflection of the sun’s light
Why is carbon monoxide bad?
-Carbon monoxide is a highly toxic but odourless
gas. It can cause death if it builds up in an enclosed
space due to faulty heating appliances.
-CO is toxic to humans as CO can from a strong bond
with haemoglobin in red blood cells. This is a stronger
bond than that made with oxygen and so it prevents the oxygen attaching to the haemoglobin.
What’s cracking?
- conversion of large hydrocarbons to smaller molecules of by breakage of C-C bonds
- High Mr alkanes smaller Mr alkanes+ alkenes + (hydrogen)
Economic reasons for catalytic cracking?
-The petroleum fractions with shorter C chains (e.g. petrol and naphtha) are in more demand than larger fractions.
• To make use of excess larger hydrocarbons and to supply demand
for shorter ones, longer hydrocarbons are cracked.
• The products of cracking are more valuable than the starting materials (e.g. ethene used to make poly(ethene), branched alkanes
for motor fuels, etc.)
Conditions for catalytic cracking?
Low pressure
High Temperature (450°C)
Zeolite Catalyst
Describe the synthesis of chloralkanes
In the presence of UV light alkanes react with chlorine to form a mixture of products with the halogens substituting hydrogen atoms.
Give the overall reaction of methane to chloromethane
CH4 + Cl2 CH3Cl + HCl
In general, alkanes do not react with many reagents.
This is because the C-C bond and the C-H bond are relatively strong
Define reaction mechanism
a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs
What’s the mechanism for the synthesis of chloroalkane?
FREE RADICAL SUBSITUTION
What are the steps in free radical subsitution?
STEP ONE: Initiation : the formation of the radicals
STEP TWO: Propagation: 2 repeated steps that build up the desired product in a side-reaction
STEP THREE: Termination: 2 radicals collide and make a stable product
Explain Step 1 in free radical substitution
-Initiation
Cl2 >2Cl. Essential condition: UV light
-The UV light supplies the energy to break the Cl-Cl bond. It is broken in preference to the others as it is the weakest.
-The bond has broken in a process called homolytic fission.
-each atom gets one electron from the covalent bond
-When a bond breaks by homolytic fission it forms Free Radicals
Define free radical?
- A Free Radical is a reactive species which possess an unpaired electron
- Free Radicals do not have a charge and are represented by a .
Explain Step 2 in free radical substitution
-Propagation
-The chlorine free radicals are very reactive and remove an H from the methane leaving a methyl free radical: CH4 + Cl. =HCl + .CH3
-The methyl free radical reacts with a Cl2 molecule to
produce the main product and another Cl free radical:
.CH3 + Cl2 =CH3Cl + Cl.
-All propagation steps have a free radical in the reactants and in the products.
-As the Cl free radical is regenerated, it can react with several more alkane molecules in a CHAIN REACTION
Explain Step 3 in free radical substitution
- Termination
- Collision of two free radicals does not generate further free radicals: the chain is TERMINATED.
-.CH3 + Cl . =CH3Cl
- .CH3 + .CH3 =CH3CH3 (Minor step leading to impurities of ethane in product. Write this step using structural formulae and
don’t use molecular formulae)
Now describe the bromination of propane
STEP ONE Initiation
Br2 ->2Br. Br2 splits in the same way as Cl2
Essential condition: UV light
STEP TWO Propagation
- Remove one H from the alkane to produce a radical : CH3CH2CH3 + Br. -> HBr + CH3CH2CH2.
- To the radical produced in the previous step add a Br CH3CH2CH2.+ Br2 -> CH3CH2CH2Br + Br.
STEP THREE Termination
CH3CH2CH2.+ Br. ->CH3CH2CH2Br
CH3CH2CH2.+ CH3CH2CH2. ->CH3CH2CH2CH2CH2CH3
What are alkenes?
- hydrocarbons containing only C and H
- are unsaturated containing at least one C=C
- general formula CnH2n
- have a gradation in physical properties
- have similar chemical properties
Describe the bonding in alkenes
-C=C double covalent bond consists of one sigma (σ)
bond and one pi (π) bond
-π bonds are exposed and have high electron density.
-They are therefore vulnerable to attack by species which ‘like’ electrons: these species are called electrophiles.
Describe the formation of pi bonds (1 sigma and 1 pi in alkenes)
-The π bond is formed by sideways overlap of two p
orbitals on each carbon atom forming a π-bond above and below the plane of molecule.
-The π bond is weaker than the σbond.
What is the shape and bond angle in alkenes?
The arrangement of bonds around the
>C=C< is trigonal planar and has the bond angle 120o.
What are stereoisomers?
Stereoisomers have the same structural formulae
but have a different spatial arrangement of atoms.
What type of isomerism can alkenes exhibit?
E-Z stereoisomerism
Why do e/z stereoisomerism arise?
(a) There is restricted rotation around the C=C double bond.
(b) There are two different groups/atoms attached both ends of the double bond.
What’s an addition reaction?
a reaction where two molecules react together to produce one
Why are alkenes relatively reactive?
because of the relatively low bond enthalpy of the pi bond.
Describe the Reaction of Alkenes with Hydrogen
Change in functional group: alkene -> alkane
Reagent: hydrogen
Conditions: Nickel Catalyst
Type of reaction: Addition/Reduction
Define electrophile
an electron pair acceptor
Describe the Reaction of alkenes with bromine/chlorine
Change in functional group: alkene dihalogenoalkane Reagent: Bromine Conditions: Room temperature (not in UV light) Mechanism: Electrophilic Addition Type of reagent: Electrophile, Br+ Type of Bond Fission: Heterolytic
-As the Br2 molecule approaches the alkene,
the pi bond electrons repel the electron pair in
the Br-Br bond. This INDUCES a DIPOLE. Br2 becomes polar and ELECTROPHILIC (Brδ+).
-The INTERMEDIATE formed, which has a positive charge on a carbon atom is called a CARBOCATION
Describe the Reaction of Hydrogen Bromide with Alkenes
Change in functional group: alkenehalogenoalkane
Reagent: HCl or HBr
Conditions: Room temperature
Mechanism: Electrophilic Addition
Type of reagent: Electrophile, H+
-HBr is a polar molecule because Br is more
electronegative than H. The H δ +
is attracted to the electron-rich pi bond.
-This reaction can lead to two products when the
alkene is unsymmetrical
-‘Markownikoff’s Rule’
In most cases, bromine will be added to the
carbon with the fewest hydrogens attached to it
-If the alkene is unsymmetrical, addition of hydrogen
bromide can lead to two isomeric products.(major and minor)
Why does an unsymmetrical alkene lead to a major and minor product?
In electrophilic addition to alkenes, the major product is formed via the more stable carbocation intermediate.
The order of stability for carbocations is tertiary > secondary >primary
In exam answers
•Draw out both carbocations and identify as primary, secondary and tertiary
•State which is the more stable carbocation e.g. secondary more stable than primary
•State that the more stable carbocation is stabilised because the methyl groups on either (or one) side of the positive carbon are electron releasing and reduce the charge on the ion.
•(If both carbocations are secondary then both will be equally stable and a 50/50 split will be achieved)
Describe the Reaction of alkenes with steam to form alcohols
-Industrially alkenes are converted to alcohols in one step. They are reacted with steam in the presence of an acid catalyst.
-This reaction can be called hydration: a reaction where
water is added to a molecule
-Reagent : steam
Essential Conditions
High temperature 300 to 600°C
High pressure 70 atm
Catalyst of concentrated H3PO4
-CH2=CH2 (g) + H2O (g) -> CH3CH2OH (l)
-The high pressures needed mean this cannot be
done in the laboratory. It is preferred industrially,
however, as there are no waste products and so has
a high atom economy. It would also mean separation
of products is easier (and cheaper) to carry out.
What’s addition polymerisation?
- during polymerisation reactions monomers have their pi bond broken; the electrons from each pi bond make a pi bond with the neighbouring carbon atom on a different monomer - so chemically connects the monomers.
What’s the industrial importance of alkenes?
The formation of polymers from ethene based monomers is a major use of alkenes.
The manufacture of margarine by catalytic
hydrogenation of unsaturated vegetable oils using
hydrogen and a nickel catalyst is another important
industrial process.
How are waste polymers dealt with?
- Separation and recycling
- Feedstock for Cracking
- Combustion for energy production
State the 4 different types of reactions of alkenes
- Reaction of Alkenes with Hydrogen (addition)
- 2.Reaction of alkenes with bromine/chlorine (electrophillic addition)
- 3.Reaction of Hydrogen Bromide with Alkenes (electrophillic addition)
4. Reaction of alkenes with steam to form alcohols (electrophillic addition).
What are alcohols?
- saturated contain only single covalent bonds
- have a general formula CnH2n+1OH
- have a gradation in physical properties
- have similar chemical properties
Properties of alcohols?
-The alcohols have relatively low volatility due to
their ability to form hydrogen bond between
alcohol molecules.
-The smaller alcohols (up to 3 carbons) are
soluble in water because they can form
hydrogen bonds with water.
-The longer the hydrocarbon chain the less soluble the
alcohol.
Primary alcohol?
Primary alcohols are alcohols where 1 carbon is attached to the carbon adjoining the oxygen
Secondary alcohol?
Secondary alcohols are alcohols where 2 carbon are attached to the carbon adjoining the oxygen
Tertiary alcohol?
Tertiary alcohols are alcohols where 3 carbon are attached to the carbon adjoining the oxygen
Complete combustion of alcohols?
In excess oxygen alcohols will burn with complete combustion the products of complete combustion are CO2 and H2O.
What is the oxidising agent that causes alcohols to oxidise?
Potassium dichromate K2Cr2O7
Describe the partial oxidation of primary alcohols
Reaction: primary alcohol -> aldehyde
Reagent: potassium dichromate solution and dilute sulfuric acid
Conditions: warm gently and distil out the aldehyde as it forms
What is the observation seen when a primary alcohol is oxidised?
The orange dichromate ion CrO72- reduces to the green Cr3+ ion
Aldehyde TIPs
an aldehydes name always ends in -al
it always has the C=O bond on the first carbon of the chain so doesn’t need an extra number
when writing the formula of aldehydes in a condensed way write CHao not COH
What is distillation?
In general it is used as a separation technique to separate an organic product from its reacting mixture.
You need to collect the distillate of the approximate boiling point range of the desired liquid.
Describe the full oxidation of primary alcohols
Reaction: primary alcohol-> carboxylic acid
Reagent: potassium dichromate solution and sulfuric acid
Conditions: use an excess of dichromate and heat under reflux (distil off product after reaction has finished).
what is reflux?
Reflux is used when heating organic reaction mixtures for long periods.
The condenser prevents organic vapours from escaping by condensing them back to liquids.
Never seal the end of the condenser as buildup of gas pressure could cause the apparatus to explode
Anti-bumping granules are added to the flask in both distillation and reflux to prevent vigorous uneven boiling by making small bubbles form instead of large bubbles.
Describe the oxidation of secondary alcohols
Reaction: secondary alcohol -> kentone
Reagent: potassium dichromate and dilute sulfuric acid
Conditions: heat under reflux
outcome: ketone
when ketones have 5+C’s it needs a number to show the position of the double bond e.g. pentan-2-one
What’s the observation of oxidation of a secondary alcohol?
orange dichromate reduces to the green Cr3+ ion
Why cant tertiary alcohols oxidise by potassium dichromate?
Because there is no hydrogen atom bonded to the carbon with the OH group
What is the reaction of alcohols with dehydrating agents?
Reaction: alchohol-> alkene
Reagents: concentrates sulfuric or phosphoric acids
Conditions: warm under reflux
Role of Reagent: dehydrating agent/catalyst
Type of Reaction: acid catalysed elimination
(removal of a water molecule from a molecule)
Substitution reactions of alcohols to form haloalkanes?
-a mixture of haloed ions with concentrated acid NaCl+H2SO4 can be used for substituting a halogen onto an alcohol
Reaction: alcohol-> haloalkane
Reagent: Concentrated sulfuric acid and sodium halide
How are haloalkanes named?
based on original alkane
prefix indicating halogen atom: fluoro, chloro, bromo, iodo
Primary Haloalkane?
1 Carbon attached to the carbon atom adjoining the halogen
Secondary Haloalkane?
2 carbons attached to the carbon atoms adjoining the halogen
Tertiary Haloalkane
3 Carbons attached to the carbon atom adjoining the halogen
substitution?
swapping a halogen atom for another atom/ groups of atoms
nucleophile ?
electron pair donator
e.g :OH- :NH3 CN-
what’s :Nu?
represents any nucleophile
always have a line pair and act as electron pair donators
attack the positive carbon atom
where does the curly arrow start?
from a lone pair of electrons or the centre of a bond
What does the rate of the substitution reaction depend on?
the strength of the C-X bond
X=halogen
the weaker the bond, the easier it is to break and the faster the reaction
which halogens are the fastest to substitute?
iodalkanes (v. reactive)
bromoalkanes
chloroalkanes
fluoroalkanes (v. unreactive)
Describe the nucleophilic substitution with aqueous hydroxide ions.
Change in functional group: Halogenalkane-> alcohol Reagent: potassium or sodium hydroxide Conditions: in aqueous solution, heat under reflux Mechanism: nucleophilic substitution Type of reagent: Nucleophile :OH-
How do you compare the rate of hydrolysis reactions?
splitting a haloalkane with water
-Water is a poor nucleophile but it can be reacted slowly with halogenalkanes in a substitution reaction use reflux or heat for more than 20 minutes
- aqueous silver nitrate is added to a haloalkane and the halide leaving group combines with silver ion to form a silver halide precipitate.
-The precipitate only forms when the halide ion has left the haloalkanes and so the rate of formation of the precipitate can be used to compare the reactivity of the different haloalkanes.
-The quicker the precipitate is formed, the faster the substitution reaction and the more reactive the haloalkanes.
-The rate of the substitution reactions depends on the strength of the CX bond.
-the weaker the bond the easier it is to break and the faster the reaction.
AGI = yellow precipitate (forms fastest)
AgBr = equals cream precipitate
AgCl = white precipitate
What are chlorofluorocarbons?
- developed for use as aerosols and in air conditioning due to low reactivity, volatility and non-toxicity.
- found to have a detrimental effect on the ozone layer in the atmosphere
- has been banned to develop alternative chlorine free compounds.
why’s the ozone layer useful
O3 good because it filters out much of the suns harmful UV radiation
it’s continually being formed and broken down in the stratosphere by the action of UV radiation
Ozone formation?
UV light causes an O2 molecule to split into free radicals : O2+UV= O+O
when the free radical hits another O2 molecule, ozone forms: O+O2=O3
the frequency of UV light absorbed=the frequency of biologically damaging UC radiation. these reactions therefore filter out harmful UV light and allow life to survive on earth.
UV light can increase risk of skin cancer and increase crop mutation.
ozone depletion?
this is the reverse of formation reaction:
this energy is supplied by UV light:
O3+UV light= O2+O
there’s a constant amount of ozone in atmosphere because of the continuous cycle of formation and depletion of ozone
destruction of ozone layer?
radicals from CFCs and NOx from thunderstorms or aircraft may catalyse the breakdown of ozone
chlorine radicals are formed in the upper atmosphere when energy from UV radiation causes C-Cl bonds in CFCs to break:
Cf2Cl2-> CF2Cl• + Cl•
the chlorine free radical atoms catalyse the decomposition of ozone due to these reactions because they are regenerated/ contribute to hole in ozone layer:
Cl•+O3-> ClO•+O2
ClO•+O3->O2+Cl•
overall equation: O3+O•->2O2
the regenerated Cl• means that it could destroy many thousands of ozone molecules
infrared spectroscopy?
-Certain bonds in a molecule absorb infrared radiation at characteristic frequencies causing the covalent bonds to vibrate.
-Complicated spectra can be obtained then provide information about the types of bonds present in the molecule.
Below 1500 cm -1 = fingerprinting
can use spectra to identify a particular functional group in wavenumber (on data sheet thank GOD AND PRAISE THA LAWWWWD)!
what’s the greenhouse effect?
- CO2 and Ch4 and H2O are all greenhouse gases which trap the earths radiated infrared energy in the atmosphere
- infrared radiation is absorbed by C=O C-H and O-H in the above
- the greenhouse effect of a given gas is dependant on it’s atmospheric concentration and its ability to absorb IR and it’s residence time.
breathalysers measure ehthanol in breath by using IR
mass spectrometry?
the peaks caused by fragment
peak with largest m/z= Mr of complete molecule called the molecular iron
