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Learn about covalent bonding, hybrid orbitals, Stereoisomers, Optical isomerism, Enantiomers, and Racemates in organic chemistry. Discover the subtle differences in physical and chemical properties of geometric isomers.
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Unit 2 Organic Chemistry and Instrumental Analysis
Revision • Be able to define the following: Homologous series Functional group Saturated Unsaturated Isomers Addition reaction Condensation reaction Hydrolysis Physical properties Chemical properties
Covalent Bonding • A covalent bond forms when atomic orbitals overlap to produce a molecular orbital. • This new arrangement of electrons and nuclei is of lower energy than before bonding occurred. Attractions and repulsions are equal. • A chemical bond can be thought of as the localisation of electrons holding together 2 adjacent positive nuclei.
On the graph above the energy required to separate the atoms is the bond enthalpy and r0 is the length of the bond. • Chemical bonds are a continuum from covalent to ionic. Electronegativity values can help to predict the type of bonding, but polar covalent bonding has many forms. This does not account for metallic bonding or exceptions like graphite.
Bonding in alkanes • When carbon has 4 single bonds it has a tetrahedral arrangement. • The bonds are covalent. Carbon has electronic structure 1s2 2s2 2p2 or to use box notation: • The 2s and 2p orbitals combine to form “sp3” hybrid orbitals. 1s 1s 2s 2p sp3 hybrid
Hybrid orbitals • sp3 hybrids are made of 1x 2s and 3 x 2p orbitals. • Electron repulsion theory agrees with the tetrahedral arrangement. • The shape is a distorted combination of an s and a p orbital.
Sigma bonds • Overlap of a half filled sp3 with a half filled s orbital creates a sigma bond. • The overlap is end on and along the axis joining the 2 nuclei. • A sigma bond can also be formed from the overlap from overlap of 2 hybrid orbitals to form a C-C single bond. • All alkanes have bond angles of 109.5° – tetrahedral angle.
Bonding in alkenes • Ethene is flat with angles of 120°. The C=C is shorter than C-C, but not double the strength. The C=C cannot rotate easily. • This time the 2s and 2 x 2p orbitals form 3 hybrid orbitals ie “sp2”. • The sp2 orbitals lie at 120° to each other and at 90° to the remaining p orbital. Overlap creates 1 x sigma and 1 x π bonds (overlap of 2 parallel sp2 hybrids).
Bonding in alkynes • Alkynes contain a Carbon to Carbon triple bond. • This is formed from SP hybridisation • i.e. 1 s and 1 p orbital overlap end on, leaving the 2 remaining p orbitals to overlap side on in different planes to create the triple bond.
Stereoisomers Often isomerism can be subtle. Stereoisomers have the same molecular and structural formulae, but the atoms have different 3D arrangements, making them non-superimposable on each other. Your hands are an example. They have the same number of digits, the same connections, but no matter how you try and turn them, you cannot make them identical.
Geometric isomers These isomers occur when there is no free rotation around a C-C bond ie this often a C=C bond. Two kinds – cis and trans. The bond is not easily broken to allow rotation.
Physical properties of geometric isomers Often have different melting points – trans higher than cis. This is due to trans isomers being able to pack more closely, therefore they have stronger intermolecular forces of attraction. Try Q4-13 pg 127-129 Scholar book 3.
Chemical properties of geometric isomers Sometimes geometric isomers can have different chemical properties. Eg butenedioic acid. The cis form arrangement allows for water to be lost under heating to form a cyclic anhydride. The arrangement of the trans form does not allow this.
Note the significant different in melting points in the cis and trans forms. Can you draw the structure of the product?
Optical isomerism Occurs when there are 4 different groups around a tetrahedral carbon atom. Called chirality – arises from a lack of symmetry. Eg a right hand glove does not fit a left hand and vice versa. The isomers cannot be lined up or superimposed on each other.
Enantiomers Each optical isomer is called an enantiomer. When the enantiomers are 50% each the mixture is called a racemic mixture or racemate. This was made famous with thalidomide – the racemate was prescribed instead of a single enantiomer. Many biological systems use only one enantiomer eg alanine and other amino acids.
Alanine enantiomers Use a modelling kit to build the enantiomers of alanine and check that they cannot be superimposed.
Polarised light Enantiomers rotate plane polarised light to the same degree in different directions. This is why they are called optically active. Racemates are optically inactive i.e. the rotation from each enantiomer cancels out, giving no overall rotation. Although they behave the same in test tube reactions, they behave differently in the presence of other chiral molecules. Eg 1 enantiomer of limonene smells of lemons, the other of oranges.
Nucleophilic substitutions Chiral compounds produce chiral products. SN2 reactions give rise to an inverted product – see later notes on SN2 reactions. SN1 reactions give rise to racemic mixtures because the nucleophile can attack from either side. Try Q22-23 pg 142 and Q25-26 pg 144 Scholar book 3.
Bond breaking Homolytic fission – 2 shared e- separate equally. Highly reactive free radicals form. Heterolytic fission – both shared e- go to 1 atom. Ions are produced. Note – pi bonds eg in alkenes and carbonyls are easier to break than sigma bonds due to the side on overlap rather than end on. This makes them important in synthesis.
Carbocations These species form during heterolytic bond fission eg The bonds undergoing heterolytic fission tend to be polar. The electrons go to the more electronegative species
Nucleophiles “nucleus seeker” Negatively charged ions eg OH-, carbanions attracted to positively charged nuclei. Atoms with partial negative charge can also be nucleophiles eg nitrogen in ammonia.
Electrophiles “electron seeker” Positively charged eg H+, carbocations attracted to negatively charged electrons. Partially positive atoms can act as electrophiles eg hydrogen atoms in water. Polar molecules can therefore have both an electrophilic and a nucleophilic centre.
Halogenoalkanes Can be called alkyl halides or haloalkanes. Fluorine gives prefix “fluoro” etc. Name longest chain. Give substituents lowest possible number. List halogens in alphabetical order. Then list other substituents eg alkyl. Try Q21 and 22 pg 32 Scholar book 3.
Types of halogenoalkanes Primary Secondary Tertiary Try Q23-26 pg33 Scholar book 3
Elimination reactions Strong base eg KOH in ethanol. OH- attacks hydrogen atom of halogenoalkane. The H-C bond breaks, creating a new C=C bond and the halide atom leaves as a halide ion. Try Q27-30 pg 35 Scholar book 3.
SN1 reactions Substitution, nucleophilic, 1st order (1 molecule involved in RDS). SN1 reactions generally only occur with tertiary halogenoalkanes, due to the stability of the carbocation intermediate.
The inductive effect • To stabilise the carbocation, alkyl groups push electron density towards the positively charged carbon atom. • This stabilises the positive charge. • Therefore SN1 tends only to happen with tertiary halogenoalkanes.
SN2 reactions Substitution, nucleophilic, 2nd order (rate depends on the concentration of both reactants). 1 step reaction, no intermediate, transition state forms. Reaction can go backwards or forwards from the transition state. TS cannot be isolated.
Products of substitution reactions Add OH- to make alcohols. Add alkoxides (RO-) in sodium and ethanol to make ethers (R-O-R). Add – to make nitriles and extend the carbon chain by 1. Add ammonia to make amines. Try Q 36-39 pg41 Scholar book 3.
Alcohols and ethers Alcohols – functional group = OH Used to make plastics, solvents, drinks, antifreeze, steroids, fuels, explosives. Ethers – functional group = R-O-R Used as anaesthetics, solvents and petrol additives. Try Q5-8 pg 47 Scholar book 3.
More on ethers Can be written as R’-O-R’’ or R1-O-R2. If R’ and R’’ are identical – symmetrical ether. Named as “dialkylether”. If R’ and R’’ different – unsymmetrical ether. Named as substituents eg methoxyethane. Smaller alkyl group is the substituent. Try Q9-12 pg 48 Scholar book 3.
Preparing ethers Reaction of metal alkoxide with monohalogenoalkane.
Reactions of ethers Slightly polar, but not enough to be attacked by nucleophiles and electrophiles. Highly flammable!! Unstable and explosive. Form hydroperoxides and peroxides on contact with air over a few days. Good solvents for polar and non-polar substances. Eg ethoxyethane easily evaporated, but exercise caution. Treat ethers with an iron salt to ensure peroxide-free solvent.
Physical properties Complete the word bank on pg 49 Scholar book 3. Then try Q13. Consider the forces of attraction between alkanes, alcohols, ethers and halogenoalkanes. Use your knowledge to answer Q14-26 pg 50-52 Scholar book 3. Due to lack of hydrogen bonding, ethers have lower bpts than alcohols. Now try the solubility questions pg 52-53 Scholar book 3.
Preparation of alcohols Traditionally by fermentation. By hydration of alkenes. Good for large scale production. By preparation from halogenoalkanes – SN1 or SN2 with OH- nucleophile. Generally only 1 product formed.
Reactions of alcohols Alcohols react with alkali metals to create metal alkoxides and hydrogen gas (similar to reaction of water with alkali metals to make hydroxides and hydrogen gas). CH3OH + Na CH3O-Na+ + H2 Metal alkoxides, RO-, are powerful bases and nucleophiles.
Dehydration of alcohols Creates alkenes. Al2O3 catalyst More generally alcohol + concentrated H2SO4 or H3PO4 gives alkenes.
Synthesis of alkenes – elimination reactions • Dehydration of an alcohol. Alcohol vapour passed over Al2O3 catalyst to produce water and an alkene. • Alternatively for less volatile alcohols the alcohol is warmed to 80°C with phosphoric or sulphuric acid. • Another method is to remove hydrogen halide from a monohalogenoalkane using KOH in ethanol. • Most reactions produce a mixture of products.
Reactions of alkenes – electrophilic addition • The C=C bond in alkenes is electron rich ie nucleophilic. It can donate a pair of electrons to an electrophile. • Catalytic addition of hydrogen • Halogens to form dihaloalkanes • Addition of hydrogen halides (HCl) • Acid catalysed addition of water
Electrophilic addition of hydrogen halides • These molecules are polar. The δ+ hydrogen atom is attacked by the π electrons and the H-Cl bond breaks heterolytically. • The Cl- ion then attacks the carbocation to create a monohalogenoalkane. • For ethene only one product can be formed as the molecule is symmetrical. However longer chain alkenes can form different products.
Markovnikov’s rule • Attack of propene could result in either 1-chloropropane (attack at the end of the chain) or 2-chloropropane (attack at the middle carbon atom). • Mostly the product is the latter. • Markovnikov’s rule states that H adds to the carbon atom with the least alkyl groups and the halogen adds to the carbon atom with more alkyl groups.
Stability of carbocations • Markovnikov’s rule can be explained by looking at the stability of the carbocations formed. • The stability is conferred by alkyl groups pushing electron density towards the positive carbon centre and therefore stabilising the positive charge.
Acid catalysed addition of water • First the double bond attacks the H+ ion. • Then the δ– oxygen atom of water attacks the carbocation. • X- from the acid then removes a proton from the water molecule, giving oxygen back the lone pair of electrons and forming HX. • See preparation of alcohols for mechanism.
Addition with halogens • Unlike hydrogen halides and water, diatomic halogen molecules are non-polar. • As the halogen molecule approaches the electron rich double bond of the alkene, the electrons of the halogen bond are repelled, creating a temporary dipole. • The addition then proceeds via an unstable cyclic intermediate, followed by nucleophilic attack of the generated halide ion.
