Chemistry - Rings, Polymers and Analysis Flashcards
Define an Arene
A hydrocarbon with a ring of 6 carbon atoms and a delocalised π system of electrons e.g. Benzene
Define an Aromatic Compound
A compound containing a benzene ring (an arene)
What evidence is there to suggest that the Kekule formula of Benzene is not correct?
1) Benzene does NOT DECOLOURISE BROMINE IN THE DARK
2) ALL THE CARBON-CARBON BONDS ARE THE SAME LENGTH c=c bonds are shorter than c-c bonds
3) HYDROGENATION of benzene is LESS EXOTHERMIC than expected (hydrogenation of trienes have ∆H = -360, benzene has ∆H of -208
What does benzene’s ∆H show about its bonding?
MORE STABLE π BONDS than expected - due to DELOCALISED ELECTRONS
How reactive is benzene compared to alkenes, and why is this?
1) LESS REACTIVE
2) π-electrons DELOCALISED - spread out more - LOW ELECTRON DENSITY - LESS ATTRACTIVE THE ELECTROPHILES
Why are all of benzenes carbon-carbon bonds the same length?
LOW ELECTRON DENSITY - DELOCALISED electrons are spread out over the ring
Why does benzene have a lower ∆H than expected?
DELOCALISATION LOWERS ENERGY OF ∏-ELECTRONS
more stable state than isolated π-bonds - more energy is needed to break them - less favourable
What is the name of the mechanism where electrophiles react with aromatic rings?
ELECTROPHILIC SUBSTITUTION
Why don’t Arenes react with electrophiles by addition?
1) LESS REACTIVE than alkenes - delocalisation lowers electron density - electrophiles are less attracted 2) SUBSTITUTION MAINTAINS THE DELOCALISED ∏ - SYSTEM - more favourable
What is the overall equation for the Nitration of Benzene?
C6H6 + HNO3 -> C6H5NO2 + H2O
What conditions are necessary for the nitration of benzene?
50-60ºC CONC NITRIC AND CONC SULPHURIC ACID
How is the electrophile formed for the nitration of benzene?
HNO3 + H2SO4 (NO2)+ + H2O + HSO4-
Describe benzene’s structure:
1) PLANAR - all atoms on same plane, all c-c bond lengths the same 2) Each C forms 3 SIGMA BONDS - 2 to adjacent carbons, 1 to H 3) each carbon has 1 electron in a P ORBITAL above and below plane 4) P orbitals overlap and form a DELOCALISED ∏ - ELECTRON SYSTEM (cloud)
Describe the mechanism of the reaction of Nitric acid with benzene:
1) Curly arrow from inner circle edge of benzene ring to N on (NO2)+ 2) a) H and NO2 both bonded to one carbon b) inner circle now semicircle, +ve charge in circle opposite bonded c c) Curly arrow from H bond to where the circle edge was below bonded c 3) products: C6H5NO2 + H+
How is the catalyst regenerated in the Nitration of Benzene?
H+ formed in reaction combines with HSO4- formed in formation of electrophile HSO4- + H+ -> H2SO4
What conditions are necessary for the Halogenation of Benzene?
HALOGEN CARRIER catalyst AlBr3 or FeBr3
How does the halogen carrier catalyst aid halogenation of benzene?
Polarises the halogen -> stronger electrophile
Give the reaction for the formation of the electrophile in the halogenation of benzene:
1) Br2 + AlBr3 Br+ + (AlBr4)-
Describe the mechanism by which the Bromination of Benzene occurs:
1) Curly arrow from circle in Benzene ring to Br+ 2) a) H and Br bonded to same carbon in ring b) semicircle formed with + inside ring opposite bonded c c) curly arrow from H bond to where ring would be by bonded c 3) C6H5Br and H+ formed
How is the Halogen Carrier Catalyst reformed?
H+ released in reaction reacts with AlBr4- formed in generation of electrophile H+ + (AlBr4)- -> AlBr3 + HBr
What state is phenol at rtp, and what is the reason for this?
Solid - large surface area= lots of van der waals forces
How soluble is phenol in water?
Slightly soluble - OH group can form H bonds, but large non-polar surface area cannot
Why is phenol more acidic than water or ethanol?
The negative charge on the O of the Phenoxide ion is delocalised into the π - system of the aromatic ring, making the ion more stable - e.g. more favourable to the dissociation of H+. Ethanol and water do not have a π-system, and so the charge cannot be delocalised
Give an equation for the reaction of Phenol with Sodium Hydroxide:
C6H5OH(aq) + NaOH(aq) -> C6H5O-Na(aq)+ + H2O(l)
What are the properties of Phenoxide Salts?
1) VERY SOLUBLE in water - ion-dipole associations -> phenol more soluble in NaOH than H2O 2) HIGH MPT - strong ionic bonds have to be broken -> phenol only has to break intermolecular forces
What bases will phenol not react with?
WEAK BASES -Carbonate (CO3)2- or Hydrogen carbonate (HCO3)- ions too weak an acid to react
In what reaction does phenol resemble alcohols?
redox REACTION WITH Na C6H5OH + Na -> C6H5O-Na+ + 1/2 H2
Why does phenol undergo electrophilic substitution much more easily than bezene?
Ring is ACTIVATED by OH group: One of oxygens LONE PAIRS is in a P-orbital -> Overlaps with delocalised π system of the ring INCREASED ELECTRON DENSITY -> ring more attractive to electrophile
What is observed when phenol reacts with bromine
1) BROMINE DECOLOURISED 2) WHITE PPT FORMED
What is the electrophile in the Nitration of an arene?
NO2+
Where on the phenol ring does electrophilic substitution normally occur, and why?
Carbons 2, 4, and 6 - INCREASED ELECTRON DENSITY
What are the uses of phenols?
1) ANTISEPTICS/ DISINFECTANT - not used so widely anymore - corrosive 2) Making plastics 3) making resins for paints
Give an equation for the electrophilic substitution of phenol with bromine:
C6H5OH (aq) + 3Br2 (aq) -> C6H5OHBr3 (s) + 3HBr (aq)
What is meant by Carbonyl Compound?
Aldehyde or Ketone
What is meant by carbonyl group?
C=O
How do the melting points of carbonyls compared to hydrocarbons of a similar size, and what is the reason for this?
Higher mpt/bpt as C=O allows for Permanent Dipole: Permanent Dipole interactions -> stronger than VDWs
How do the boiling points of carbonyls compare to alcohols of similar sizes?
Lower Bpt - Permanent Dipole: Permanent Dipole interactions are weaker than Hydrogen Bonds
How soluble are Carbonyls in water?
small non - polar carbonyls - quite soluble Delta negative oxygen can form H bonds with water
How would you prepare an Aldehyde?
Oxidation of a primary alcohol CH3CH2OH + [O] -> CH3CHO + H2O
How would you produce an aldehyde experimentally?
Heat with Aqueous Acidic Potassium Dichromate Distillation
How would you prepare a ketone?
Oxidise a secondary alcohol under reflux with aqueous acidic potassium dichromate CH3CHOHCH3 + [O] -> CH3COCH3 + H2O
Why is the C=O Bond polar?
Oxygen is much more electronegative than Carbon
What substance is used in the test for the presence of a carbonyl compound?
2,4 - DINITROPHENYLHYDRAZINE YELLOW/ORANGE PRECIPITATE
What substance is used to identify the presence of an aldehyde?
TOLLENS REAGENT (AgNO3 in aqueous NH3) SILVER MIRROR
Give the equations to represent the reaction of an aldehyde with Tollens’ Reagent:
RCHO + [O] -> RCOOH } Aldehyde OXIDISED to carboxylic acid Ag+ + e- -> Ag } Ag+ reduced
What substance is used to form alcohols from carbonyl compounds?
SODIUM TETRAHYDRIDOBORATE, NaBH4
Give an equation for the formation of an alcohol from an aldehyde:
RCHO + 2[H] -> RCH2OH
What properties must a nucleophile have in order to undergo nucleophilic substitution with a carbonyl?
LONE PAIR and (ideally) negative charge to be attracted to the delta positive carbon
Describe the mechanism of the 1st stage of the nucleophilic substitution of Ethanal with NaBH4:
1) Curly arrow from lone pair on :H- to delta +ve charged C (nucleophile donates lone pair of electrons, forming a bond) 2) Curly arrow from double bond to O with delta -ve charge ( electronegative O takes both shared electrons from π bond 3) CH3CH2O-
Describe the mechanism for the second part of the nucleophilic substitution of ethanal with NaBH4:
1) curly arrow from lone pair on :O- to H on H-O-H 2) curly arrow from H-O bond to O on Water 3) CH3CH2OH + H-O- formed
What is the overall reaction for the nucleophilic substitution of Ethanal?
CH3CHO + H- + H2O -> CH3CH2OH + OH-
Are carbonyls reactive with electrophiles, or nucleophiles?
Nucleophiles (nucleophilic substitution)
How does the boiling point of esters compare to: 1) Carboxylic acids 2) Hydrocarbons of a similar size?
1) LOWER BPT - CANNOT FORM H Bonds 2) HIGHER BOILING POINT - C=O bond allows PERMANENT DIPOLE:PERMANENT DIPOLE INTERACTIONS
What is the general formula of an ester?
n-O-C(=O)-m
Why do unsaturated fats tend to have lower melting points than saturated fats?
the C=C double bonds make the fatty chain less flexible and so are less able to pack together efficiently in a solid
What are the conditions for margarine production?
Bubble hydrogen gas through oil high temp nickel catalyst
what is a side effect of partial hydrogenation?
some of the naturally occurring cis alkene groups isomerise to form trans groups
What are amines?
Organic compounds related to ammonia, with one or more of the hydrogens replaced by an organic group
How would you prepare simple Amines?
HEAT a HALOGENOALKANE under PRESSURE with EXCESS ETHANOLIC AMMONIA (NH3 dissolved in Ethanol)
What is the name of the mechanism of the reaction that forms simple amines?
Nucleophilic substitution - :NH3 acts as a Nucleophile
How can you form Secondary and Tertiary Amines from the reaction that forms non-aromatic primary amines?
EXCESS HALOGENOALKANE Primary amines formed in the initial reaction still have a lone pair and so can also react with leftover halogenalkane the secondary amine formed also has a lone pair, and can also undergo nucleophilic substitution to form a tertiary amino
How would you prepare an aromatic amine? give any necessary conditions
Reduction of nitroaromatic compounds - e.g. Nitrobenzene + 6[H] -> Phenylamine + 2H2O TIN and CONCENTRATED HCl = reducing agent heat under reflux
Describe and explain the difference in boiling points between primary amines and alkanes of a similar size:
Boiling points of Amines = HIGHER than alkanes Amines can form HYDROGEN BONDS which are STRONGER than Van der Waals
Describe and explain amines solubility in water:
1) SMALL AMINES = VERY SOLUBLE - H BONDS 2) Larger amines - slightly soluble - LARGE NON-POLAR AREA
Why are amines weak bases?
can use the lone pair of electrons on N to accept a proton (accept a dative covalent bond to a hydrogen ion) weak as reaction is reversible
Give an equation for Amines dissolving in water:
CH3NH2 + H2O CH3NH3+ + OH- Forms alkaline solution in water
Give a reaction for an amine reacting with an acid:
CH3NH2 + HCl -> CH3NH3+Cl-
What are Azo dyes?
Aromatic compounds with rings linked by the AZO GROUP, -N=N-
What is the first step in the formation of Azo Dyes?
Aromatic amine and NITROUS ACID react together BELOW 10*c
How is the Nitrous Acid formed for the formation of Azo Dyes?
SODIUM NITRATE and DILUTE HYDROCHLORIC ACID react 2H2O and a diazonium ion is formed (- N2+)
What happens in the Second step of forming an Azo Dye?
Diazonium ions in solution are added to PHENOL IN AQUEOUS ALKALI -usually NaOH- (C6H5O-) conditions: still temp<10*c Forming an azo dye: a phenol ring and another aromatic ring joined by -N=N-
What name is given to the useful products of azo dyes?
DYESTUFFS
What is the General Formula of an alpha-amino acid?
H2NC(R)(H)COOH
What does the R represent in the general formula of an alpha amino acid?
An organic group, of which there are 20 variants in the amino acids used in cellular protein synthesis.
Why can enzymes selectively catalyse the formation of a particular optical isomer?
All* amino acids have a chiral centre, and so have optical isomers. In biology, only ONE OF THE OPTICAL ISOMERS IS USED IN PROTEIN SYNTHESIS, so proteins (e.g. enzymes) have a built in handedness, an therefore are able to catalyse the formation of a specific isomer
What is a Zwitterion, and how is it formed?
Amino Acid With COO- and NH3+ group (2 ionic parts with opposite charges in the same species) The H+ on the COOH is donated to NH2
How does the melting points of amino acids compare to that of amines, and what is the reason for this?
MUCH HIGHER MELTING POINT Solid amino acids exist ONLY AS ZWITTERIONS GIANT IONIC STRUCTURE FORMED - very strong electrostatic attractions must be broken
How soluble are amino acids in water?
Very soluble - ionic groups can form ion-dipole interactions with water
Is an amino acid acidic or basic?
Weakly Acidic - COOH AND Weak Base - NH2
What charge does an amino acid have at a Low pH? (acidic)
+H3N-C(R)(H)-COOH O- accepts a proton to form COOH
What ion is formed of an amino acid when it is at intermediate pH?
+H3N-C(R)(H)-COO-
What ion is formed when an amino acid is at High pH? (alkali)
H2N-C(H)(R)-COO- (Extra H+ donated to solution to neutralise it)
What is an Isoelectric Point?
The pH at which the AVERAGE OVERALL CHARGE on a molecule of the amino acid is ZERO
What effect does extra Amine groups on a amino acid have on Isoelectric Point?
MORE NH3+ -> HIGHER ISOELECTRIC POINT more alkaline conditions needed to neutralise to NH2
What are proteins?
Polymers formed by CONDENSATION reactions of amino-acids, linking COOH with NH2 of a different amino acid (Peptide linkage) eliminating water
What name is given to 2 amino acids joined together?
DIPEPTIDE
What is the general repeat unit of a protein?
H -(-N-C-C=O) ) \n
Give a general formula for the Acid Hydrolysis of a protein:
H2NCHRCONHCHRCOOH + H2O -> H2NCHRCOOH + H2NCHRCOOH
What is the ionic equation for the acid-catalysed hydrolysis of a protein?
+H3N…COOH + H2O + H+ -> +H3NC(R)(H)COOH + +H3C(R)(H)COOH (Amide group is charged, Carboxylic group is not)
What conditions are necessary for the acid-catalysed hydrolysis of a protein?
HEAT UNDER REFLUX 6Mol/dm3 AQUEOUS HCl
How do proteins maintain an 3D structure?
Held together by non-covalent interactions (e.g H bonds)
What names can be given to this Group? | C=O | H-N |
1) AMIDE GROUP - for general molecules 2) PEPTIDE LINK - for linking 2 amino acids
What is the ionic equation for the Hydrolysis of a protein under alkaline conditions?
H2N…COO- + OH- -> H2N..COO- + H2N..COO-
How soluble are esters in water?
slightly soluble - cannot form h bonds, but can form weaker permanent dipole:permanent dipole interactions
Give an equation for the formation of an ester from a carboxylic acid:
CH3COOH + CH3OH -> CH3COOCH3 + H2O
What type of reaction is the formation of esters from carboxylic acids, and what conditions are necessary?
1) CONDENSATION 2) CONC SULPHURIC ACID - HEAT under REFLUX
What is the general formula of an Acid Anhydride?
R1COOCOR2
How is an acid anhydride formed?
DEHYDRATION of carboxylic acid CH3COOH + CH3OOH -> CH3COOCOCH3 + H2O
Give an equation for the formation of an ester from an acid anhydride:
R1COOCOR1 + R2-OH -> R1COOR2 + R1COOH
What conditions are necessary for the esterification of an acid anhydride
Gentle heat- no catalyst
What are the advantages of using acid anhydrides for the formation of esters, compared to carboxylic acids?
1) MILDER CONDITIONS - the sulphuric acid catalyst could react with other functional groups in the molecule as well 2) HIGHER YIELD - anhydride reaction is irreversible, carboxylic acid reaction is irreversible
Give a reaction for the alkali-catalysed hydrolysis of an ester:
R1-COO-R2 + HO- -> R1COO- + R2-OH ester + base -> Carboxylic acid SALT + Alcohol
What conditions are necessary for the alkali-catalysed hydrolysis of an alkali?
REFLUX (heat) AQUEOUS NaOH
How would you convert the salt formed in the alkali-catalysed Hydrolysis of an ester into its parent carboxylic acid?
1) NEUTRALISE with aqueous acid 2) DISTILL of carboxylic acid
Give an equation for the acid-catalysed hydrolysis of an ester:
R1-COO-R2 + H2O -> R1-COOH + R2-OH
What conditions are necessary for the acid-catalysed hydrolysis of an ester?
REFLUX AQUEOUS HCl
Why is base catalysed hydrolysis generally preferred over acid-catalysed hydrolysis?
1) Alkali catalysed hydrolysis (under given conditions) is IRREVERSIBLE - HIGH YIELD acid catalysed is reversible
What are esters used for?
FLAVOURINGS PERFUMES
What are triglycerides?
esters of GLYCEROL
What is glycerol’s systematic name, and what is its displayed formula?
1) PROPAN-1,2,3-TRIOL 2) OH OH OH | | | H2C—-CH—-CH2
What are Fatty Acids?
Carboxylic Acids that react with glycerol
What are Unsaturated compounds?
Compounds with a C=C double bond
What are Polyunsaturated compounds?
compounds with many c=c double bonds
What is the formula for reading the shorthand names of fatty acids?
c,d(p1…p(d)) c = no. of carbons d = no. of double bonds p1..p(d) = carbon position of double bonds
What is the shorthand name for Cis,cis - 3,7 octadienoic acid?
8, 2 (3,7)
What is the longhand name for 10, 3 (2,5,8)?
2,5,8 Decatrianoic acid
What health issues are linked to saturated and unsaturated fats?
1) CIRCULATORY, HEART and OBESITY problems - too much sat. fat 2) excess BAD CHOLESTEROL produced by high intake of sat. fat - deposited onto artery walls - increased risk of HEART DISEASE 3) TRANS FATS - high bad cholesterol
What is the name of the reaction that forms biodiesel?
TRANSESTERIFICATION
What is the equation that forms biodiesel?
fatty acid + 3(short alcohol) -> Biodiesel (ester) + Glycerol
Give an example of a catalyst used in the formation of biodiesel:
NaOH (or other bases)
What are advantages of biodiesel?
1) burning BIOFUELS is CARBON - NEUTRAL - no overall increase of CO2 in the atmosphere 2) Decreases consumption of fossil fuels - combats climate change 3) Can be used on its own (usually blended with normal diesel) 4) RENEWABLE - can always grow more plants for oil/alcohol - can be formed form leftover cooking oil
What is a disadvantage of using Biodiesel?
1) Land is taken up - deforestation - lack of land for food production
What is an addition polymer?
A polymer formed from the addition of many alkene molecules
give the equation for the addition polymerisation of CH2=CHX:
n(CH2=CHX) -> -[-C(H)2-C(H)(X)-]- n
What are the advantages of using addition polymers?
Many applications -e.g. polythene Inexpensive - formed from - alkenes - byproduct of cracking
What is the main disadvantage of Addition polymers?
NON-BIODEGRADABLE - will remain in landfill for a very long time
What type of polymer is one with the linkage: O || -C-O-
POLYESTER, which is a type of condensation polymer
What normally are the monomers of a polyester?
DIOL and DICARBOXYLIC ACID HO-n-OH and HOOC-m-COOH
Give a general equation for the formation of a polyester:
n(HO-x-OH) + n(HOOC-y-COOH) -> -[-O-x-OOC-y-COO-]-n + 2n(H2O)
What are polyesters used for?
FIBRES FOR CLOTHING
What is a disadvantage of using polyesters made from diols and dicarboxylic acids?
NON-BIODEGRADABLE - long lifetime in landfill
What methods are used to dispose of Addition polymers, and what are their drawbacks?
1) BURNING -produce TOXIC FUMES 2) RECYCLING - sorting needed - expensive and labour intensive 3) CRACKING for use as feedstocks and fuels
What is an example of a biodegradable polyester?
POLYLACTIC ACID -[-O-C(CH3)(H)-C(=O)-]-n
What makes polyesters, such as polylactic acid, different to those formed from diols and dicarboxylic acids?
has ONE MONOMER, which contains BOTH OH and COOH GROUPS
What are the uses of polylactic acid?
1) SOLUBLE STITCHES 2) BIODEGRADABLE WASTE SACKS
How are polyamides formed?
creating AMIDE LINKAGES between monomers - reaction of amine groups with carboxylic acid groups n(HOOC-x-COOH) + n(H2N-y-NH2) -> -[-C(=O)-x-C(=O)-NH-y-(H)N-]-n + 2n (H2O)
What are polyamides used for?
Making fibres - e.g. nylon
What is the difference between normal polyamides and proteins?
Proteins have ONE MONOMER - amino acids
How may you chemically degrade condensation polymers? -e.g. polyesters and polyamides-
HEAT under REFLUX with CONCENTRATED AQUEOUS ACID/ALKALI (reactions same as hydrolysis for amines/esters)
Why is Poly(lactic acid) useful?
MORE SUSTAINABLE - it is BIODEGRADABLE, and RENEWABLE as monomer is produced easily form corn starch
What word is give to describe the breakdown of condensation polymers due to UV light, and why does this happen?
PHOTODEGRADATION C=O bond absorbs radiation, causing a reactive excited state in which the ester or amide linkage is easily broken
what is a tertiary amine?
3 other groups bonded to the N - no H -N
What is a secondary Amine?
Amine which has 2 carbons bonded to the Nitrogen -NH
What is a primary Amine?
An amine which has one carbon bonded to the nitrogen -NH2 group
How are amines classified?
according to how many carbons are bonded to the nitrogen in the molecule
What is the stationary and mobile phase in Thin Layer Chromatography?
1) Stationary = Solid - silica 2) mobile = liquid - organic solvent
What is the purpose of carrying out Chromatography on a small scale?
Analysis
What is the purpose of carrying out Chromatography on a large scale?
Purification
what is the formula for Rf value?
distance travelled by the component ——————————————- distance travelled by solvent
what factors affect the rate at which the component moves up the tlc? (and therefore Rf)
1) How strongly the component is adsorbed 2) Solubility - more soluble = faster moving
How are the components separated in TLC?
adsorbption
what are the advantages of TLC?
1) quick 2) inexpensive
Describe the method of gas chromatography:
1) sample injected into GC machine and VAPORISED 2) Sample carried into column by INERT gas carrier - passes over S.P. 3) the sooner the sample is ADSORBED(solid) or DISSOLVED(liquid) onto the stationary phase the longer the retention time - depends on: a) volatility of compound b) how strongly compound interacts with stationary phase
what is a limitation of TLC?
molecules of a similar size and functional groups poorly separated
What is the stationary and mobile phase in Gas Chromatography?
1) Stationary = liquid or solid 2) mobile = gas
Why may compounds have similar retention times?
Similar functional groups - similar interactions with column/mobile phase
What does the area under a peak in a gas chromatogram indicate?
The amount of each compound
What is GC-MS?
Gas Chromatography combined with Mass Spectrometry
Describe the process of GC-MS:
1) Substance sent through a gas chromatography machine 2) Peaks are detected in the chromatogram 3) Components sent to a mass spectrometer 4) Generated Mass Spectra compared with Spectral databases using a computer
What are the advantages of GC-MS?
1) HIGH RESOLUTION - possible to resolve complex mixtures 2) Can operate with SMALL AMOUNTS of sample 3) Due to large database, very likely positive indication of individual components
What are the limitations of GS-MS?
1) Difficult to identify similar compounds - similar retention times 2) Unknown/uncharacterised compounds have to reference retention times/spectra for identifying 3) EXPENSIVE equipment
What are the applications of GC-MS?
1) Forensics - drug testing - small traces can be analysed in detail 2) testing for organic pollutants - pesticides 3) Airport security - detecting explosives 4) Space missions - finding substances to indicate traces of life
How does Infra-red spectroscopy work?
Molecules absorb IR radiation, the energy causes the covalent bonds in the molecule to VIBRATE at different frequencies dependent on the functional groups. this different frequencies are then picked up on the spectra
What peak indicates the Mr of a substance on a Mass Spectra?
MOLECULAR ION PEAK (furthest peak to the right)
What name is given to all other peaks in mass spectra?
FRAGMENT ION PEAKS
What must be remembered about mass spectra fragments?
MUST HAVE +VE CHARGE
What are the 2 types of NMR spectroscopy?
13C and 1H
How do 13C nuclei give NMR peaks?
Different 13C nuclei in DIFFERENT CHEMICAL ENVRIONMENTS absorb at different frequencies, giving different peaks
What term is given to 13C or 1H nuclei in identical chemical environments, and what does this mean?
EQUIVALENT - give the same peak
What does the number of peaks in a 13C NMR spectrum indicate?
The number of carbons with different environments in the molecule
What gives the reference peak in 1H NMR, and where on the spectrum is this shown?
TETRAMETHYLSILANE (TMS) - Si(CH3)4 (4 equivalent carbons, 12 equivalent protons) 0ppm
Why does ‘Splitting’ occur in 1H NMR?
The energy gap between the spin states of one proton is slightly affected by the spin states of protons attached to NEIGHBOURING carbons - the protons are COUPLED
What factor affects the splitting pattern?
How many protons are attached to the ADJACENT carbons in the molecule
What rule can be used to describe the splitting pattern?
N+1 RULE - an extra peak is added for every proton on an adjacent carbon
What names are given to the first 4 types of splitting?
Singlet - 1 peak - 0 adjacent protons Doublet - 2 peaks - 1 adjacent protons Triplet - 3 peaks - 2 adjacent protons Quartet - 4 peaks - 3 adjacent protons
What are the exceptions to the n+1 rule?
1) OH PEAKS NEVER SPLIT other protons and never are split themselves 2) EQUIVALENT PROTONS NEVER SPLIT EACH OTHER e.g. CH3-CH3 only gives one peak, with no splitting, as all the protons are equivalent
How would you identify a -OH or -NH peak on an NMR spectrum?
1) Run the spectrum normally for reference 2) Add D2O to the sample - O-H -> OD, -NH -> -ND 3) Run spectrum again - Peaks that have disappeared indicate the presence of -OH or -NH
Why can’t pure phenol be run through an NMR machine, and how is this resolved?
NMR spectra can only be obtained for liquid samples Solvent needed
What factors must be considered when determining a suitable solvent for NMR?
1) If a compound is in SOLUTION, the solvent must not give rise to peaks that will interfere with the spectrum 2) For proton NMR, the solvent cannot contain protons, or all protons must be deuterated - wont give an NMR signal
Give 2 examples of common deuterated solvents:
1) D2O 2)CDCL3
Give a practical example of use of NMR spectroscopy:
1) MRI - Magnetic Resonance Imaging - diagnosing location of tumours A person is placed in a strong magnetic field - NMR signals are detected from different regions of space within the body, usually from magnetic 1H nuclei in water molecules, which give different signals dependent on the envrionment
