Organic Functional Groups Lecture 1 PDF

ORGANIC FUNCTIONAL GROUPS Dr Graeme Kay Teaching Group Leader – Chemical Sciences 01224 262548 [email protected] C X Bond polarity is crucial when considering the properties, stability and reactivity of molecules. Alkyl Groups By convention the symbol R is an alkyl group. The simp...

ORGANIC FUNCTIONAL GROUPS Dr Graeme Kay Teaching Group Leader – Chemical Sciences 01224 262548 [email protected] C X Bond polarity is crucial when considering the properties, stability and reactivity of molecules. Alkyl Groups By convention the symbol R is an alkyl group. The simplest is the methyl group, CH3 We can replace 1 or more H atoms of the methyl group with further alkyl groups Primary (1º) alkyl group Here we replace 1 H by another alkyl group. H RCH2 e.g. CH3CH2 CH3 C H Secondary (2º) alkyl group Here we replace 2 H atoms by alkyl groups. R2CH e.g. (CH3)2CH CH3 CH3 C H  CH3 CH3 C CH3  ALKANES General formula: CnH2n+2 H H H Often H C H H C C H referred to as H H H saturated ethane hydrocarbo methane ns Physical Properties There are no polar bonds present in alkanes so they cannot undergo strong intermolecular attractions. The low mol. mass alkanes (1-C to 4-C ) are gases at room temperature (only van der Waals interactions). As we increase the C number there are more forces of attraction and they become liquids (5-C to 20-C) Solubility Again, because of the lack of polarity, alkanes are totally insoluble in water. (remember we need to make bonds between the molecule e.g. ion dipole or H-bonds or dipole-dipole bonds). Alkanes are therefore soluble in other molecules with which they can interact e.g. other alkanes and lipids. The word lipid incorporates many compounds that are insoluble or immiscible with water. So alkanes, fats and oils etc. can be classified as lipids. For this reason alkanes are absorbed into lipid material in the body and are not soluble in aqueous blood. Important for substances to enter, for example, the brain from the bloodstream e.g. general anaesthetics. Chemical Reactivity and Stability Since we only have non-polar C-C and C-H bonds alkanes are very stable and inert. (used to be called paraffins from the latin parum affinis meaning “little affinity”). Storage Conditions Can store safely in almost all conditions of light, heat, moisture and air.  ALKENES eneral formula: CnH2n So we have a double bond present H H H CH3 Often referred C C C C to as an H H H H unsaturated ethene propene hydrocarbon We commonly have 2 or more double bonds in the molecule Remember we can have isolated or conjugated double bonds – e.g. the pentadiene molecules: Physical Properties Form Like alkanes, there are no polar bonds present in alkenes so they cannot undergo strong intermolecular attractions. The low mol. mass alkenes (2-C to 4-C ) are gases at room temperature (only van der Waals interactions). As we increase the C number there are more forces of attraction and they become liquids (5-C or more) Solubility Again, because of the lack of polarity, alkenes are totally insoluble in water (remember we need to make bonds between the molecules e.g. ion dipole or H-bonds or dipole-dipole bonds). Alkenes are therefore soluble in other non-polar molecules like lipids e.g. alkanes, fats and oils. Chemical Reactivity and Stability Alkenes may be very similar to alkanes in their physical properties. They are very different in terms of their reactivity. The reactive functional group is the double bond. The reaction that characterises alkenes is the addition reaction A B + A B C C C C Of significance is when A-B is a hydrogen H-H (H2) molecule. The product is then: H H C C So the alkene has been reduced to an alkane. We have converted an unsaturated structure to a saturated one. This is a commonly used process in to convert oils (contain long chains with double bonds) to fats (mainly saturated chains). Another reaction of this type is called a hydration reaction when water can add to the double bond. This is particularly possible in the presence of an acid catalyst. We shall look at the mechanism of this reaction since it illustrates so many aspects of organic chemistry. Firstly, the acid catalyst H+ is an electrophile – it seeks electrons for its stable valence shell: Remember, carbocations are also electrophiles seeking electrons. These electrons are provided by a nucleophile, in this case water: This will finally lose H+ to give the product, an alcohol. H C C O H Note about carbocations Carbocations have different stabilities (and so ease of formation) So when a mechanism requires the formation of a carbocation the process will take place with the following ease: 3º easiest since most stable – easily formed 2º less stable 1º unstable – much less likely to be formed So, in the hydration of propene, we can get the formation of two carbocations : OXIDATIONS One type of oxidation that is common in biochemical pathways is epoxide formation: O C C C C Epoxides are very reactive. They become protonated (with an H+ ion) and are then attacked H by nucleophiles: OH O Nu C C Nu C C Another type of oxidation that takes place in air is the formation of a peroxide: O O C C C C Peroxides are very unstable and often are explosive! Reactivity of conjugated alkenes When we saw the A-B addition of a molecule to an A alkene B we saw the following: + A B C C C C Because of the delocalisation of electrons, if we reacted butadiene with A-B we find the following: B H H H A H C C H C C C C H C C H H H H H H H B A H C C C C H H H H So we obtain a mixture of products instead of a single product. This is very common with delocalised structures. Sharing the same formula as the alkenes, CnH2n are the cyclic derivatives of alkanes, the cycloalkanes. cyclopentane cyclohexan e These cyclic derivatives behave like the open-chain alkanes in their stability/reactivity. The exception is cyclopropane Behaves more like an unsaturated molecul Storage requirements of alkenes The formation of peroxides is a major problem so store in well sealed containers  ALKYL HALIDES General structure: R-X PHYSICAL PROPERTIES Although we have a polar bond between the C of the alkyl group and the halogen we still only have weak van der Waals bonds So the low RMM compounds are gases/liquids at room temperature SOLUBILITY Again we are unable to form significant bonds with water so alkyl halides are insoluble in water. They are commonly used as solvents themselves: CHCl3 CH2Cl2 chlorofor dichloromethan m e Reactivity and Stability Under normal conditions alkyl halides are relatively stable. However, they are very important for the synthesis of other functional groups. Remember the C-X bond is polar which sets up partial charges in the bond δ+ δ– C X The δ+ carbon can then attract nucleophiles (with their unbonded electrons) So these compounds can undergo substitution reactions called nucleophilic substitution reactions. They are of the type: Depending on the nucleophile the product is of a particular functional group: e.g. when Nu: is – OH the product is an alcohol when Nu: is – OCH3 the product is an ether. Remember neutral molecules e.g. water act as nucleophiles The mechanism of the substitution depends on the nature of the alkyl halide. If we have a 3° structure R it ionises first due to Rstable 3° carbocation C R C X R R R sp3 hybridised C sp2 hybridised C tetrahedral planar Using 3-D representations, we have: The nucleophile can attack from either side: If the 3 R groups were different we would have 4 different groups attached to the central C atom. The 2 products are mirror images of each other In drugs this feature very often means one will be active and the other inactive (or toxic). They can interact with targets differently. This type of reaction is called an SN1 reaction (substitution, nucleophilic and the rate depends only on the concentration of the substrate i.e. alkyl halide). With 1° and 2° structures we get a transition formed instead of an ionisation: The Xthis With atom then leaves reaction and we get the orientation substituted of the bonds to theproduct. central C (i.e. the configuration) is inverted. This may also be very significant on biological activity etc. The rate depends on the concentration of both the substrate and nucleophile and is called SN2 reaction. Other main points of nucleophilic substitution reactions: Not just alkyl halides undergo these reactions – many other functional groups also do this The atom or group that is substituted is called the leaving group – there are many types of leaving group They compete with another reaction type – the elimination reaction Elimination reactions occur when heated with bases (B:) to give alkenes: Nucleophiles also have unbonded electron pairs like bases – hence the competing reactions. Therefore a major use of alkyl halides is in the synthesis of other functional groups. The other main use is as a solvent. Storage Alkyl halides are reasonably stable in air but chloroform can form a highly toxic compound called phosgene. O C Cl Cl Conjugated and isolated dienes have different chemical and spectroscopic behaviour. The difference is due to the way the p orbitals overlap to form the π bonds of the double bonds. In conjugated systems all of the adjacent p orbitals can overlap. H H H C C C C H H H The concept of conjugation is important when considering the reactivity of a molecule. Molecules that contain conjugated π bonds tend to be more stable and less reactive than those with isolated π bonds. We are not restricted to alkenes when looking at conjugated molecules. e.g. we can have conjugated carbonyl containing groups and also groups called ene-ynes. Ultraviolet (UV) Spectroscopy UV – a method of structure determination applicable specifically to conjugated systems. When a conjugated molecule is irradiated with UV light, energy absorption occurs and a π electron is promoted from the HIGHEST OCCUPIED MOLECULAR ORBITAL (HOMO) to the LOWEST UNOCCUPIED MOLECULAR ORBITAL (LUMO). As a general rule the greater the extent of conjugation, the less energy needed (i.e. the longer the wavelength of radiation required)  Why are some compounds coloured and some not? Benzene is a colourless – and is a conjugated system -Carotene is orange – and is a conjugated system -Carotene Visible region of electromagnetic spectrum is adjacent to UV (~400- 800nm) Coloured compounds have EXTENDED systems of CONJUGATION Their UV absorptions extend into the visible region. When white light hits, wavelengths between 400-500nm are absorbed and all other wavelengths are emitted – we see white light with blue removed hence the yellow/orange colour of -Carotene What molecules are conjugated?