Transcript File
Homologous Series (family) • Each series has a general formula. • All members possess the same functional group. It is the functional group that gives the series its characteristic reactions. • Members of the same homologous series have similar chemical properties and methods of preparation. • The chemical formula increases by CH2 from one member to the next up the series. • There is a gradual change in physical properties from one member to the next. The most common example of this is the increasing melting and boiling points as we go up a series. The reason for this is the increasing London forces as the molecules get larger. Alcohols Important characteristics of the alcohols homologous series; • The names of all alcohols end in ‘-ol’ • The functional group of the alcohols is –OH (hydroxyl group) • The first 3 alcohols are polar and therefore are ________ in water. • All alcohols follow the general formula; Three categories of alcohols; • Primary - The carbon attached to the -OH group is directly bonded to only one alkyl group. (i.e. 1 carbon) • Secondary - The carbon attached to the -OH group is directly bonded to two alkyl groups. (i.e. 2 carbons) • Tertiary - The carbon attached to the -OH group is directly bonded to three alkyl groups. (i.e. 3 carbons) Isomers and Naming Isomers can result from both chain branching and varying the position of the -OH group. 1. In naming, the main chain (longest) must contain the -OH group, whose position is indicated by a number. 2. Number the chain to give any branches the lowest possible number. 3. Name the branches: methyl (-CH3), ethyl (C2H5), propyl (-C3H7) etc. Example 2 – Draw and name the 4 isomers of C4H9OH Oxidation • In carbon chemistry, oxidation can mean either adding oxygen or removing hydrogen. • This is often referred to as increasing the oxygen to hydrogen ratio. • Full oxidation occurs during combustion • Combustion of alcohols (in excess oxygen) produces carbon dioxide and water. Useful Oxidation Summary Chemicals used to oxidise alcohols and their results (learn names and colours) 1. Acidified potassium permanganate solution (KMnO4); During the reaction the purple permanganate (MnO4-) ion is reduced to colourless Mn2+ ions. 2. Acidified potassium dichromate solution (K2Cr2O7) During the reaction the orange dichromate ion (Cr2O72– ) is reduced to green Cr3+ ions. 3. Copper (II) oxide and heat The black oxide is reduced to red copper metal during the reaction. Aldehydes and Ketones • Many common flavours from different foods are caused by molecules within the foods called aldehydes and ketones. • Both of these molecules contain the same functional group (the carbonyl group) and are named in similar ways. Aldehydes (Alkanals) • Aldehydes are produced via the oxidation of primary alcohols • The names of all aldehydes end in ‘-al’ • The functional group of the aldehydes is C=O (carbonyl group) • The carbonyl group is always attached to the end carbon for the aldehydes. • The main industrial use for aldehydes is in the production of thermosetting plastics. Ketones (Alkanones) • Ketones are produced via the oxidation of secondary alcohols • The names of all ketones end in ‘-one’ • The functional group of the ketones is C=O (carbonyl group) • This carbonyl group is never attached to the end carbon in the ketones (and is usually indicated via number.) • The main industrial use for ketones is as solvents and varnishes (propanone is the solvent used for nail varnish remover) How to distinguish between aldehydes and ketones As both homologous series have the carbonyl group (in many reactions) they react in similar ways. Aldehydes and ketones with the same number of carbons are isomers of one another. As the carbonyl group is found on different carbon positions we can distinguish between them. Aldehydes oxidise whereas ketones do not. Chemicals (and results) used to oxidise aldehydes 1.Benedict’s solution Blue solution Orange-red precipitate Cu 2+ ions reduced to Cu2O i.e. copper (I) oxide 2.Tollen’s reagent i.e. AgNO3(aq) + NH3(aq) A ‘silver mirror’ is formed Ag+ ions reduced to Ag atoms 3.Acidified potassium dichromate solution Orange solution Green solution Cr2O7 2− reduced to Cr 3+ ions Carboxylic Acids (Alkanoic Acids) • Carboxylic acids are produced via the oxidation of aldehydes • The names of all carboxylic acids end in ‘-oic acid’ • The functional group of the carboxylic acid is -COOH (carboxyl group) • This carboxyl group is always attached to the end carbon • Carboxylic acids are polar and therefore dissolve in water • The O-H part of the carboxyl group provides hydrogen bonding (see bonding) Making Esters Esters are compounds formed by a condensation reaction between alcohols and carboxylic acids. In a condensation reaction two molecules join and a small molecule (often water) is removed. Naming Esters The name of an ester indicates the alkanol and acid which go into making it. • The first part is derived from the alcohol: -anol becomes -yl. (i.e. ethanol becomes ethyl) • The second part is derived from the carboxylic acid: -oic acid becomes -oate. (i.e. ethanoic becomes ethanoate) Antioxidants • Antioxidants are molecules that play an important role in preventing our food from spoiling too quickly by stopping oxidation reactions from taking place. • The antioxidant molecules are reducing agents, they cause other substances to be reduced while being oxidised themselves. • One of the simplest antioxidants is vitamin C. (see ‘free radical’ section of notes for more detail) https://www.youtube.com/watch?v=QM3lMKoT6U0 From Plants From Animals Sunflower Oil Beef fat Olive Oil Cod liver Oil Vegetable Oil Pork Fat (lard) Walnut Oil Butter The main difference between fats and oils is that; • fats are normally solid at room temperature • oils are normally liquid at room temperature Oils have a lower melting point than their fat counterparts due to the greater amount of unsaturation within the (oil) molecules. The absence of a double bond allows the fat molecules to be more regularly ‘tuning fork’ shaped and consequently the fat molecules can fit into one another. If a double bond is present then the oil (and some fats) molecules ‘zigzag’ and the molecule chains become distorted and cannot fit into one another. Molecules which can pack closely together due to their regular structure have stronger London forces between the molecules and thus higher melting points. Therefore fats have higher melting points than oils – and fats are solid at room temperature. Fats in the Body The main function of fats and oils is to provide energy. Fats and oils release about twice the amount of energy of carbohydrates. Fats/oils release their energy carbohydrates (think sugar rush!) more slowly than Fats and oils also help provide the body with vitamins as vitamins are soluble in fats/oils. Margarine manufacturers are required to add some of these vitamins to their products to prevent certain vitamin deficiency problems. The structure of fats and oils Fats and oils are actually special forms of esters where the alcohol, glycerol (propane-1,2,3-triol) has three hydroxyl groups. Glycerol is termed a ‘trihydric alcohol’. Glycerol can therefore make ___ ester links when reacting with long carboxylic acids (fatty acids.) 1 glycerol reacts with ____ acids Since glycerol is constant, it is in the acid chain that we potentially find the double bond – if there is a double bond the acid is called an alkenoic acid. Fatty acids are saturated or unsaturated straight chain carboxylic acids with even numbers of C atoms ranging from C4 to C24, but mainly C16 to C18. Bromine can be used to distinguish between saturated and unsaturated molecules. (unsaturated fats/oils will decolourise bromine.) The above fat/oil molecule is called a triglyceride. When a fat or oil is formed, the glycerol molecule can react with up to three different fatty acid molecules. Any particular fat is made of a mixture of different triglycerides, so no fat or oil is a pure triglyceride. Turning oils into fats It is possible to convert oils into fats. (eg ‘Bertolli’ olive oil spread) This takes place via a process known as ‘hardening’. Hardening is an addition reaction (hydrogenation) where the unsaturated carbon double bonds are converted to saturated single carbon bonds. Margarines are made by partial hydrogenation of oils using a nickel catalyst. The amount of hydrogenation can produce margarines with different properties. Soaps Soaps are made via alkaline hydrolysis of fats/oils. The alkali used is usually sodium hydroxide or potassium hydroxide. The fatty acid forms as the sodium or potassium salt. These salts are then ‘salted out’ of the reaction mixture by adding a great excess of sodium chloride and the soap can then be filtered off. Soaps and detergents are known as emulsifiers (or ‘emulsifying reagents’.) This simply means that they allow oils and water to become permanently mixed. Sodium (or potassium) salts of long chain fatty acids have two separate parts in terms of bonding – a long hydrocarbon chain ‘tail’ (which is non polar) and an charged ionic ‘head’ (from the alkali.) When detergent or soap is added to oil and water, the tail goes into the oil while the head stays in the water. Oil Water and detergent Amines Important characteristics of amines homologous series; • The functional group of the amines is –NH2. This is called the amine or amino group. • The N-H groups provides hydrogen bonding (see bonding) • Small amines are polar and therefore dissolve in H2O Proteins The element of nitrogen is essential in food chains and it is found in the form of proteins. Proteins are the molecules which make up our muscle fibres, hair, nails, skin, enzymes, hormones etc. Proteins are generally very large molecules which are made up from smaller molecules called amino acids. Protein are naturally occurring polymers. Amino acids (monomers) contain two functional groups – the amine group (-NH2) and the carboxyl group (-COOH) As amino acids contain (-COOH) and (-NH2) groups then they can react as both an acid or as an alkali. The number of possible amino acid structures is very great, but nature only uses 26 different structures. Essential amino acids cannot be made by the body and must be obtained from our diet. Protein molecules normally consist of several thousand amino acids condensed together so the permutations are endless! (Hence the huge variety of protein structures.) Proteins are made via condensation polymerisation of amino acids. The link formed between the amino acids is called a peptide link (also known as an amide link) Proteins are also referred to as poly(peptides) or poly(amides) Type of Proteins Proteins in the body perform a vast range of jobs. As a result, they exist in a range of sizes and shapes. These polar peptide links can hydrogen bond with each other in the same molecule or with different molecules (as shown in above example.) Fibrous; Fibrous proteins form the structural materials in animal tissues – e.g. skin, muscle, hair, nails. Globular; Globular proteins tend to have spiral chains folded and twisted round into more compact units. E.g. Enzymes, hormones and haemoglobin. Enzymes Enzymes are biological catalysts. Enzymes are said to be specific – i.e. each enzyme has a particular job/function. Enzymes work via the ‘lock and key principle’ The shapes of the molecules are influenced by the presence of hydrogen bonds between the chains. Enzymes are most active within certain narrow temperature and pH ranges. (optimum) The protein structure of the enzyme is permanently altered at high temperature or low pH conditions as the hydrogen bonds are broken. This is called ‘denaturing’ the protein. During denaturing, the enzyme changes shape but covalent bonds are not broken. Hydrolysis of Protein Like all condensation polymers, proteins can be hydrolysed back into their amino acid building blocks. In the lab this can be achieved through refluxing the protein with concentrated acid however this happens more efficiently in the stomach during digestion via enzymes. The amino acids produced by the breakdown of proteins can be identified by using the technique of chromatography. Terpenes Terpenes are key components of the essential oils of many types of plants and flowers. Essential oils are a mixture of organic molecules and are used widely as natural flavour additives for food, as fragrances in perfumery, and in medicine and alternative medicines such as aromatherapy. Essential oils are concentrated extracts of the volatile, insoluble (in water) aroma compounds from plants. Synthetic terpenes have greatly expanded the variety of aromas used in perfumery and flavours used in food additives including creating the distinctive smell of many spices. Structure of Terpenes Terpenes are unsaturated compounds formed by joining together 2 methylbuta-1,3-diene (isoprene) units. https://www.youtube.com/watch?v=sA34OoZBQOE https://www.youtube.com/watch?v=IGCagIgV85g Oxidation of Terpenes Chemists have found that terpenes can be oxidised to form new compounds which have different properties from the original terpene. Similarly the reverse (reduction) can occur. (just like alcohols, aldehydes and carboxylic acids etc…) The Volatility of Molecules • In chemistry and physics, volatility is the tendency of a substance to vaporize. • Vaporize means to change directly from a solid into a vapour without first melting. • The volatility of a molecule can be predicted from its size and the functional groups present. (think bonding…) • In general the lower the boiling point, the higher the volatility. UV light • Ultraviolet (UV) light is a high energy form of light and is present in sunlight. • When molecules become exposed to UV light they vibrate and their bonds break. These are known as photochemical reactions. • Sunburn and skin aging are caused by broken bonds. Suntan lotion prevents the UV light reaching the skin. • When UV light breaks bonds free radicals are produced. Free Radicals Stable molecules have paired electrons. Free radicals are unpaired electrons and are therefore very reactive. Free radical chain reactions have the following steps: 1. Initiation 2. Propagation 3. Termination. Free Radical Scavengers • Due to the detrimental effect of free radicals on the skin and body, cosmetic companies have started adding free radical scavengers to their products. • A free radical scavenger is a molecule which can react with free radicals to form stable molecules and prevent chain reactions • Food also contains free radical scavengers in the form of ___________.