Organic Chemistry The fact that carbon can form a wide variety of relatively stable long chain molecules results in this very important branch of Chemistry: Organics. Carbon forms strong covalent bonds with itself and other atoms (see SS). Why are these compounds called Organic What is a Hydrocarbon? Questions: P167 Read 166-167
Types of Formula Look at PP168-169 of the text book and try to define the following (use butane as an example in each case) Empirical formula Molecular formula Structural Formula Displayed formula Skeletal formula P169 Q1-2
*Not really obvious with the alkanes and molecules can have more than one functional group The organic molecules are organised into groups with similar features called Homologous series Members of the group have: the same general formula the same functional group* similar chemical properties a gradual variation in physical properties Name General formula Eg. P171 Q1-2
Structural Isomerism Same molecular formula different structural formula C4H10 could be drawn: Challenge: Draw all of the possible variations of the displayed formula for C5H12
The Alkanes The Organic Chemistry this year deals with aliphatic compounds (fatty due to their fatty or oily properties). Aromatic (due to their smell) compounds are studied in Y13. The alkanes are the most basic homologous series, and provide the basis for the nomenclature of all aliphatic compounds. General formula: C n H 2n+2 Write the molecular formulae for the first 10 and name n = 7-10: NB1 CH2 is known as the homologous increment each new member increases by this much. NB2 The alkanes and cycloalkanes are saturated hydrocarbons - why?
The alkyl groups are the alkanes with one less hydrogen, and they can attach to other molecules as side chains to form branch chain molecules, eg. *name the molecule from the end which gives the smallest numbers in the name * no. of C atoms 1 2 3 4 5 root name meth eth prop but pent branch name methyl ethyl propyl butyl pentyl formula CH 3 C 2 H 5 C 3 H 7 C 4 H 9 C 5 H 11
Try to draw the displayed formulae of the structural isomers of octane and give them their systematic name. P177 Q1-2
Where do we get the alkanes from? Crude oil undergoes fractional distillation, and the fractions produced can be subjected to: - Cracking - Reformation Read pages 178-179 of your text book and define these terms, giving details of the process and examples. P179 Q1-2
Alkanes are: c/less gases from n = 1 to 4 c/less liquids from n = 5 to 15 white waxy solids n = 16 Try to explain this trend Bpts and Mpts of the Alkanes
Reactions of the alkanes 1. Combustion Egs? Exothermic reactions that form CO 2 and H 2 O in XS O 2, but CO and C otherwise. NB. If the alkane is well mixed before ignition, the reaction is explosive. Read PP180-181: Carbon monoxide (toxic), oxides of nitrogen and sulfur (acidic), carbon particulates and unburned hydrocarbons, are formed during the combustion of alkane fuels What problems do these substances cause? How do we combat this problem in internal combustion engines?
2. Substitution reactions between alkanes and halogens The hydrogen atom(s) in the alkane are replaced by halide atoms. Rate of reaction: with F 2 > Cl 2 > Br 2 > I 2 (can be explosive with the 1st two halogens). Rates decrease with longer chain alkanes. These are photochemical reactions, and happen at room temperature with the presence of ultra-violet light (sunlight will do). The energy provided by the UV light is sufficient to break the Cl-Cl bond and initiate the reaction. Demo: Hexane and bromine
Methane and chlorine Hydrogen atoms in the methane are replaced one at a time by chlorine atoms. You end up with a mixture of chloromethane, dichloromethane, trichloromethane and tetrachloromethane, making it an unsuitable method for preparing a specific haloalkane. Complete... Steamy fumes observed.
Mechanism: This is a Free Radical Substitution Reaction A species (either an atom or a group of atoms) is a free radical if it contains a single unpaired electron. a chlorine radical a methyl radical The initial step involve the homolytic fission of a covalent bond The chlorine radical goes on to break other covalent bonds in a chain reaction.
requires energy to break the bond always produce free radicals, and the reaction can continue
Free radicals come together and the reaction stops P185 Q1-2
The alkanes as fuels and the search for alternatives Read PP182-183
The alkanes as fuels and the search for alternatives Read PP182-183 What are biofuels? Give examples of Biofuels? Why are they a good idea? Explain Carbon Neutrality P183 Q1-2
Bonding and Hybrids The four electrons in carbon s 2nd shell are not equivalent, but the bonds formed with other species are identical. How? Forming single covalent bonds Before carbon bonds to 4 hydrogen atoms to form eg. methane, the three 2p orbitals of the carbon atom are combined with its 2s orbital to form four new orbitals called "sp 3 " hybrid orbitals. These are equivalent. This process is endothermic, but the energy released by the subsequent formation of bonds more than compensates.
These hybrid orbitals have 75% p-character and 25% s-character which gives them a shape that is shorter and fatter than a p-orbital. The new shape looks a little like... These sp 3 orbitals overlap with the 4 1s orbitals of the hydrogen atoms to form σ bonds. By VSEPR theory, these bonds repel to a maximum of 109.5º
Forming double covalent bonds First, one of the 2s electrons is promoted to the empty 2p orbital. Then, the 2s, 2p x and 2p y orbitals combine to form sp 2 hybrids which bond with 3 other atoms in a trigonal planar arrangement (120º). The 2p z orbital with a single electron forms a π bond with the neighbouring C atom. This bond resists rotation, keeping the other bonds in the same plane. NB. Pi bonds form between many different atoms, Eg... P187 Q1-2
The classification of organic reactions depends on the mechanism involved. In general, there are 2 types of species which attack to form new covalent bonds. These are: Nucleophiles Electron rich species with a lone pair of electrons that can be donated to other species with positive, electron deficient centres. Electrophiles Electron deficient species which are attracted to electron rich species, accepting lone pairs and forming new covalent bonds.
The Alkenes Alkenes contain a carbon-carbon double bond (they are unsaturated). General formula: CnH2n n=2 Ethene n=3 Propene
Now it gets more interesting: alkenes with 4 or more carbon atoms in them show structural isomerism.
Geometric (cis-trans or E/Z*) isomerism When the species are on the same side of the C=C double bond, the isomer is given the prefix cis. If on the other side of the bond, the assignation is trans. Names? cis-but-2-ene (Z)-but-2-ene trans-but-2-ene (E)-but-2-ene NB. This is a form of Stereoisomerism *This is sometimes called the CIP system
With simple molecules, (Z) will correspond to cis, and (E) to trans but the old system lacked definition when referring to more complex molecules it was designed to distinguish between simple geometric isomers. Consider these geometric isomers...
How does geometric (cis-trans or E/Z) isomerism come about? The carbon-carbon double bond consists of a sigma bond and a pi bond. It cannot rotate without breaking the pi bond.
The other condition... The species on both of the carbon atoms involved in the double bond must be different. Geometric isomers have the same chemical properties but their physical properties differ (mpts and bpts).
The reactions of alkenes Alkenes obviously combust, but the more significant reactions are with: H 2 X 2 H-X KMnO 4 Themselves You need to memorise reagents, products and conditions (temp, press and catalyst) The Π bond is attacked (by an electrophile*), and breaks, allowing the electrons to bond the two carbon atoms to other things. These are addition reactions the mechanism for the reaction is known as electrophilic addition. *Remember, an electrophile is an electron deficient species that seeks out electron rich centres
1. Hydrogenation Alkenes react with hydrogen in the presence of a finely divided metal (nickel) catalyst at high temperatures. The corresponding alkane is produced. This is not really a useful reaction. Why? When is this reaction used?
Describe how the following alkanes can be formed from a suitable alkene giving chemical equations Propane Butane Pentane 2-methylpentane 2-methylpropane.
2. Alkenes with halogens H 2 C = CH 2 (g) + Cl 2 (g) ClCH 2 CH 2 Cl (l) Structural: CH 2 = CH 2 (g) + Br 2 (aq) BrCH 2 CH 2 Br red/brown colourless Structural: A liquid alkene can be added to bromine liquid or to bromine dissolved in an organic solvent (CCl 4 or volasil). If the alkene is a gas, it can be bubbled through bromine water. Important: The decolourisation of bromine seen in the second reaction is a test for an alkene. Expts: Cracking of alcohols and alkanes Expts: Cracking of alcohols and alkanes
Draw structural formulae and show how bromine reacts with: Propene But-2-ene Pent-1-ene 4-methylpent-1-ene 2-methylpropene.
3. Alkenes with hydrogen halides The reactivity of these compounds: HF < HCl < HBr <HI This is due to the relative bond strengths and affects reaction rates. These electrophilic addition reactions take place with HX in the gas phase or in non-aqueous solvent (to avoid dissociation).
Show how HBr reacts with ethene, propene, but-2-ene. Name the products in these reactions and show the states of the species. Show how HI reacts with but-1-ene and 3-ethylpent-1-ene. Name the products in these reactions.
4. With manganate(vii) The alkene is oxidised in aqueous acidic or alkaline manganate (VII) Acidified potassium manganate(vii) is decolourised by an alkene and a diol is formed. 5CH 2 = CH 2 + 2H 2 O + 2MnO 4 - + 6H + 5HOCH 2 CH 2 OH + 2Mn 2+ ethane -1,2 -diol Draw the structural formulae for the organic molecules. The same basic reaction occurs with alkaline potassium manganate(vii), but the solution turns brown as manganese(iv)oxide is formed. Suggest how to make propane-1,2-diol, butane-2,3-diol and 3- methylpentane-2,3-diol. PQ1
Eg. Ethene and HBr Mechanisms What kind of bond fission is this? Stability of Carbocations
Eg. Propene with HX P190/193 Questions
5. Polymerisation Alkenes undergo addition polymerisation where the same molecules join to each other to form chains. Eg. Ethene forms polyethene ( polythene ). OR Repeating unit P195 Q1-2
POLYMERS: Solutions, problems and solutions Read PP194-195 Why are polymers so important as modern materials? What problems are caused by their use? Explain some solutions to these problems What is life-cycle analysis?