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A2 Chemistry MATERIALS REVOULTION (MR) BONDING AND STRUCTURE  ELECTRONEGATIVITY (AND TRENDS)ELECTRONEGATIVITY (AND TRENDS)  PREDICTING POLARITY OF.

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Presentation on theme: "A2 Chemistry MATERIALS REVOULTION (MR) BONDING AND STRUCTURE  ELECTRONEGATIVITY (AND TRENDS)ELECTRONEGATIVITY (AND TRENDS)  PREDICTING POLARITY OF."— Presentation transcript:

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2 A2 Chemistry

3 MATERIALS REVOULTION (MR) BONDING AND STRUCTURE  ELECTRONEGATIVITY (AND TRENDS)ELECTRONEGATIVITY (AND TRENDS)  PREDICTING POLARITY OF BONDS AND MOLECULESPREDICTING POLARITY OF BONDS AND MOLECULES  INTERMOLECULAR FORCES OF ATTRACTIONINTERMOLECULAR FORCES OF ATTRACTION  EXPLAIN/PREDICT THE EFFECT OF TEMPERATURE, CRYSTALLINITY AND CHAIN LENGTH ON POLYMER PROPERTIESEXPLAIN/PREDICT THE EFFECT OF TEMPERATURE, CRYSTALLINITY AND CHAIN LENGTH ON POLYMER PROPERTIES  POLYMER SOFTENING ABOVE Tm, BRITTLE BELOW TgPOLYMER SOFTENING ABOVE Tm, BRITTLE BELOW Tg  MODIFYING POLYMER PROPERTIES, COPOLYMERISATION, PLASTICISERSMODIFYING POLYMER PROPERTIES, COPOLYMERISATION, PLASTICISERS  PROPERTIES OF ALL MATERIALS DEPENDS ON STRUCTURE AND BONDINGPROPERTIES OF ALL MATERIALS DEPENDS ON STRUCTURE AND BONDING REACTION MECHANISMS  EXPLAIN NATURE OF AMINO GROUP (NH ₃⁺)EXPLAIN NATURE OF AMINO GROUP (NH ₃⁺) ORGANIC FUNCTIONAL GROUPS  AMINES AND AMIDES AMINES AND AMIDES  SYSTEMATIC NOMENCLATURE IN NAMING ALIPHATIC PRIMARY AMINES AND DIAMINESSYSTEMATIC NOMENCLATURE IN NAMING ALIPHATIC PRIMARY AMINES AND DIAMINES ORGANIC REACTIONS  ADDITION/CONDENSATION POLYMERISATIONADDITION/CONDENSATION POLYMERISATION  STRUCTURAL FORMULAE OF COND. POLYMER FROM GIVEN MONOMERSSTRUCTURAL FORMULAE OF COND. POLYMER FROM GIVEN MONOMERS  ACID AND ALKALINE HYDROLYSIS OF ESTERS AND AMIDESACID AND ALKALINE HYDROLYSIS OF ESTERS AND AMIDES  ACID NEUTRALISATION OF AMINEACID NEUTRALISATION OF AMINE  ACYLATION TO FORM AN AMIDEACYLATION TO FORM AN AMIDE  PURIFYING ORGANIC SOLID PRODUCT BY RECRYSTALLISATIONPURIFYING ORGANIC SOLID PRODUCT BY RECRYSTALLISATION APPLICATIONS  GREENER CHEMISTRYGREENER CHEMISTRY  RECYCLING/HAZARDOUS WASTE/CARBON FOOTPRINTRECYCLING/HAZARDOUS WASTE/CARBON FOOTPRINT

4 ELECTRONEGATIVITY CHEMICAL IDEAS 3.1, 5.3 AND 5.4 START -- ELECTRONEGATIVITY

5 ELECTRONEGATIVITY ELECTRONEGATIVITY IS THE MEASURE OF THE TENDENCY OF AN ATOM OF AN ELEMENT TO ATTRACT BONDING ELECTRONS There are several ways of working out electronegativity values but all give similar relative values and provide us with a similar order of elements. The most common way of measuring electronegativity refers to the Pauling scale, created by Linus Pauling. In this way, we see that the elements with the highest electronegativities are those that are found in the top right of the periodic table (excluding the noble gasses) and those with the lowest are found in the opposite corner, the bottom left. From this we deduce that the atoms in molecules that are most likely to have highest electronegativities are those that are small, reactive and non-metal, whereas those with the lower electronegativity values are likely to be reactive metals with large atoms. Using this information, we can generally say that when we find a fluorine atom covalently bonded to a carbon atom, the bond will be polar because there is a large difference in electronegativity across the bond (Fluorine has E/NEG value of 4.0 whereas carbon has a value of 2.6). In this way, we can use electronegativity values to help us determine the polarity of a bond. In reality, polar bonding lies somewhere between covalent and ionic; it’s a type of covalent bonding with ionic characteristics. We say that the more polar a bond is, whether that be due to electronegative differences or not, the more ionic character it has. Small differences in electronegativity i.e. between carbon (2.6) and hydrogen (2.2) show so little ionic character that we say for all intents and purposes that the bond is in fact covalent.

6 ELECTRONEGATIVITY TABLE

7 ELECTRONEGATIVITY AND BOND POLARITY The reason we can use electronegativity values to predict bond polarity is simple, when we think about the it’s definition; the tendency of an atom to attract bonding electrons. We already know from earlier study that electrons exist in clouds around nuclei but now we must consider their distribution. In a non-polar bond i.e. between atoms in the gas H ₂, electrons will be overall evenly distributed and so neither one of the atoms will possess a δ+ (delta positive/small positive) or δ- charge, ignoring the occurrence of instantaneous dipoles. However when one of these bonding atoms attracts the electrons in the covalent bond more strongly, this atom will accumulate a δ- (small negative) charge, due to the negative charge that electrons possess. As such, the other atom will be slightly positively charged and the bond between the atoms will display a difference in polarity. This having been said, there is more than just electronegativity that affects bond polarity. The shape of a molecule/size of atoms also has an effect on whether or not a bond will be polar. Going back to our example of H ₂, we know that as each of the bonding atoms are exactly the same size, each nucleus is exactly the same distance from the bonding electrons. When this is not the case i.e. between H and Cl in HCl, one atom (Cl in this case) has it’s nucleus further from the bonding electrons than the other and so effectively has a weaker pull on them. In HCl, this information alone would lead us to believe that the H atom would become slightly negative as it’s nucleus is much closer to the bonding electrons however, we must also consider atomic core charges. The atomic core of an atom is everything but the outer shell of electrons, so chlorine has an atomic core charge of +7 overall [+17 (nucleus) -10 (electrons in all but the outer shell)]. As hydrogen has only one electron and one proton, there is no atomic core charge and so considering all the above (including electronegativity values) we can deduce that the H-Cl bond is in fact polar, with chlorine harbouring a δ- charge and hydrogen having a δ+ charge. HH NO OVERALL POLARITY H Cl δ-δ-δ+δ+ POLARITY

8 INTERMOLECULAR FORCES OF ATTRACTION (IMFAs) CHEMICAL IDEAS 5.3, 5.4 AND 5.8 START -- IMFAS

9 IMFAs From earlier study we know that there are various forms of intermolecular attraction. Permanent dipoles exist in molecules where substantially different electronegativities leave atoms with constituent delta charges (hydrogen bonding is a strong form of this). Instantaneous dipoles arise from the unpredictable and random nature of electrons about a nucleus; they may be unevenly distributed across an atom or molecule at any given time, creating a temporary instantaneous dipole within said atom or molecule. Induced dipoles happen when the electron cloud around an atom is influenced to become asymmetric in the presence of other electrons, for example an instantaneous dipole occurring in one of the two Cl atoms in Cl ₂, causing the electron could in the other spread unevenly, thus inducing another dipole. As the weakest of the intermolecular forces of attraction, instantaneous dipole - induced dipole bonding occurs in all molecules and can even happen in ones with a permanent dipole. The effects of this attraction are most noticeable when we look at the noble gasses, where no other form of bonding occurs. It is also here that we notice the importance of atomic radii; xenon has a higher boiling point than helium despite both diatomic molecules only exhibiting id-id bonding. This is because xenon atoms have a much larger region of electron density surrounding them and so there are much more electrons to create a larger polarity. The larger distance between the outer shell and the nucleus also means that those in the outermost shells are less attracted and so are easily influenced by other atoms’ electrons; the atom is said to be more polarisable. These stronger IMFAs mean that xenon has a higher boiling point than helium.

10 IMFAs These id-id attractions also depend on chain length and branching. In general, longer, straight chain molecules (hydrocarbons in particular) have higher boiling/melting points because there are more opportunities for these attractions to occur and also less branching means greater contact between the molecules. This closeness allows stronger id-id bonding to occur. Molecular dipoles also have conditions under which they work. Permanent dipoles exist when bonding atoms have largely different electronegativities. Molecular dipoles occur when permanent dipoles within the molecule leave it with an overall dipole. For example; a water molecule consists of two permanent dipoles between O and H that create an overall dipole within the molecule. H H O δ-δ- δ+δ+ δ+δ+ δ+δ+δ-δ- POLARITY However, there are molecules such as tetrachloromethane that despite having many (4) dipoles, have no overall dipole. In these cases, it is the tetrahedral arrangement of chlorine around the central carbon atom that leave it with no charge. In the permanent dipoles, chlorine harbours a delta negative charge, leaving the carbon with a delta positive. When four chlorines are all leaving the carbon with a delta positive charge, the charge becomes superimposed on the carbon atom and the polarities are almost “cancelled out”. THERFORE, BOND POLARITY DEPENDS ON ELECTRONEGATIVITIES WHEREAS MOLECULAR POLARTIRT DEPENDS ON THIS AND THE SHAPE OF THE MOLECULE

11 BONDING AND STRUCTURE A substance’s properties are determined by it’s bonding and structure; the way in which atoms are held together and the way in which they are arranged relative to one another (respectively). The properties of a solid substance will be determined by the types of particles within the solid, the way in which these are held together and lastly, the overall structure and way in which the particles are arranged relative to one another. In a solid, the particles may be ions, molecules or atoms each of which affect the properties differently. For example; ionic solids conduct electricity then molten or dissolved and ionic/polar molecules may dissolve in water. With regard to the way in which particles are held together; they can be held ionically, covalently, metallically or via weak intermolecular bonds like id-id bonding. In a solid, atoms can all be strongly bonded to one another, like in SiO ₂, which has a very high melting and boiling point and is strong and hard as a result. Conversely, atoms may be held together strongly in a molecule, like in CO ₂, but the molecules aren’t held together very strongly (weak intermolecular bonds), resulting in a low melting and boiling point (it’s a gas at room temp). The final structure of a solid also helps determine how hard or flexible it is. With 1D solids like the chains in polythene, solidity falls behind flexibility, whereas with 2 and 3D shapes like the giant covalent network structures of some silicon and carbon allotropes are very hard and gritty.

12 BONDING AND STRUCTURE IONICCOVALENT NETWORK METALLIC COVALENT MOLECULAR MACROMOLECULAR STRUCTURES GIANT LATTICE (GOES ON INDEFINITELY) MOLECULAR, MADE OF ATOMS

13 POLYMERS CHEMICAL IDEAS 5.6 AND 5.7 START -- POLYMERS

14 POLYMER PROPERTIES The characteristics of polymers that affect their properties are; 1.Chain length - longer chains means more opportunity for IMFAs to occur, stronger polymer, higher melting and boiling points. 2.Side chains - more electronegative side groups allow for stronger IMFAs to occur, stronger polymer, higher melting and boiling points. 3.Stereoregularity - side groups positioned at regular intervals that are orientated in a specific way allow for closer packing of chains and so stronger IMFAs, stronger polymer, higher melting and boiling points. 4.Branching - less branching means chains lie closer together, allowing stronger IMFAs to occur, stronger polymer, higher melting and boiling points. 5.Chain flexibility - increasing the rigidity of hydrocarbon chains by modifying side groups (i.e. add benzene) can lead to increased strength, stronger polymer, higher melting and boiling points. 6.Cross-links - extensive linkage via covalent bonds greatly higher melting and boiling points. Increasing the chain length of a polymer generally increases it’s strength but a critical length must be reached before this actually comes into effect. With hydrocarbons this is about 100 repeating units, with nylons it’s 40. The tensile strength of the polymer is a measure of how much pressure it can take before snapping. An increase in tensile strength is attributed to longer chains offering more points of contact between neighbouring chains and so more IMFAs and an overall larger attraction between chains. Longer chains also tangle together more. Both of these structural points mean that the chains are less likely to slide over one another and so are less flexible.

15 POLYMER PROPERTIES Cooling and heating polymers gives us different products to what we might usually expect from a solid and this is due to the fact that they contain both amorphous (random) and crystalline (ordered ) sections. Cooling a polymer will freeze the amorphous regions as they stand and so when any pressure is applied they change position by breaking/shattering. They are said to have become glassy/brittle. At these lower temperatures, the IMFAs have more control over the properties than at higher temperatures. Heating the polymer when in this state will allow chains to move relative to each other again, the temperature required for this to happen is called the glass transition temperature (T g ) and describes the point at which the polymer moves from its brittle, glass phase into the flexible, elastic phase. In this phase, the polymer will have some of the characteristic we expect from it (flexibility and other typical plastic polymers). Further heating causes it to leave the elastic phase and enter the liquid phase. The temperature required for this to happen is the melting temperature (T m ) and indicates that the crystalline structures in the polymer have now broken as well, giving way to a viscous fluid. TEMPERATURE STRENGTH glasselasticliquid With polymers, the term crystalline refers to the areas within the polymer that are arranged in a regular way. Many polymers exhibit both crystalline and amorphous regions that allow the polymer a certain degree of flexibility (chains may slide over one another in amorphous regions). Polymers with regular chain structures are the ones most likely to have extensive crystalline arrangements as stronger IMFAs may form between chains due to them being closer. So to summarise; polymers with a high proportion of crystalline arrangements are more likely to be stronger as strong IMFAs hold chains rigidly (less sliding). As the flexibility of a polymer is determined by the ease at which chains may slide over each other, those with a high proportion of amorphous regions are more likely to be flexible.

16 MODIFYING PROPERTIES TO SUIT NEEDS We use polymers for a variety of different things and so we must be able to modify their properties to suit their purpose. Copolymerisation involves introducing a new monomer with a similar structure but perhaps an additional side group. This will effect the ability of the chains to fit together as closely and so will effect the chain’s flexibility and strength. Adding a plasticiser is another way in which we may modify the properties of a polymer. Sometimes called a “molecular lubricant”, a plasticiser is a small addition that sits between polymer chains and increases the distance between them. It therefore makes it easier for chains to slide over one another and so increases the flexibility of the polymer. Di-(2-ethyhexyl) hexanedioate is a common plasticiser for PVC. Cold-drawing utilises the fact that all polymers contain crystalline regions. Cold-drawing (stretching) polymer chains creates a neck in which chains like up to form a more crystalline region, the process continues until all but the pulling ends are crystalline. This process therefore creates much stronger fibres that are much less flexible. KEY PLASTICISER POLYMER CHAIN

17 ADDITION AND CONDENSATION POLYMERISATION Addition polymerisation involves the formation of a polymer from many monomers, commonly containing C=C bonds. When different monomers are used, the process is called copolymerisation. As it only involves the joining of monomers there is a 100% atom economy. Condensation polymerisation involves many monomers joining together to form a polymer and a smaller stable molecule, like HCl or water. The two monomers must have at least two suitable functional groups for a polymer to form (i.e hydroxyl and carboxylic acid).

18 AMINES AND AMIDES CHEMICAL IDEAS 13.8 START -- AMINES AND AMIDES

19 NAMING AMINES Amines are the organic chemistry relatives of ammonia. An amine may be either primary, secondary or tertiary depending on whether only one, two or all three of the hydrogens have been replaced with an alkyl (hydrocarbon) chain. Simple amines are named with the suffix “-amine” like methylamine, dimethylamine and trimethylamine (rotting fish smell). Higher homologues are names with the prefix “-amino” like 2-aminopropane. Diamines exist and are named like the above, for example; 1,6-diaminohexane. The properties of amines are similar to those of ammonia, as they both have a nitrogen atom with a lone electron pair. This makes amines and ammonia; very soluble in water, a base, a,ligand and a nucleophile. Smaller amines are soluble but the longer alkyl groups in larger homologues disrupt hydrogen bonding between water molecules and the amine so they’re not soluble.

20 AMINES AS BASES, LIGANDS AND NUCLEOPHILES Ammonia will react with water to form an ammonium ion and OH ⁻ ions, as the lone pair of electrons on the nitrogen forms a dative bond with H ⁺ ions to form NH ₄⁺ ions. As hydroxide ions are also produced, ammonia has a basic nature in solution. Ammonia and amines also react with acids, the H ₃ O ⁺ ions in the acidic solution acting as more powerful proton donors, making their reaction with amines go to completion, eradicating their profound smell. For example, ethylamine will react with H ₃ O ⁺ ions to form an ethylammonium ion (CH ₃ -CH ₂ -NH ₃⁺ ) and water. This is amine neutralisation. The lone pair of electrons on the nitrogen atom means that amines, just like ammonia, can form coordinate bonds with metal ions to form complex ions. The [Cu(NH ₃ ) ₄ (H ₂ O) ₂ ]² ⁺ ion forms a deep blue solution, just like if butylamine were to be added as a ligand; [Cu(CH ₃ -CH ₂ - CH ₂ - CH ₂ -NH ₂ ) ₄ (H ₂ O) ₂ ]² ⁺. Both also share a similar structure. Amines may also act as nucleophiles, just as ammonia does, due to the lone pair of electrons on the nitrogen. This lone pair will be attracted to the electrophilic carbon atom in halogenoalkanes and will disrupt and break the bond between the carbon and the halide to form, at first, a cation of R-NH ₃⁺, then a standard secondary amine. Such substitution reactions may also occur with secondary amines, to produce a tertiary amine (as well as the H ⁺ and Hal ⁻ ).

21 AMIDE FORMATION AND HYDROLYSIS Amides are formed when ammonia or amines react with an acyl chloride. This reaction, as with most involving the highly reactive acyl chloride species, is very vigorous and would not happen in aqueous conditions as the AC would react with the water to form a carboxylic acid. It can be conducted at room temperature and must be in anhydrous conditions. Ammonia reacts with the AC to form a primary amine, like ethanamide (ethanoyl chloride + ammonia), whereas amines react to form secondary amides. Primary amides are named from the root carboxylic acid (which becomes the AC). Hydrolysis of the amide link requires water and either an acid or alkaline catalyst. Both require heating under reflux and moderate concentrations of catalyst in solution, but produce different products. Acid catalysis produces a carboxylic acid and ammonium ion/alkylammonium ion (due to presence of protons in solution). Alkaline catalysis produces a normal amine but also a carboxylate salt, as the H from the COOH group dissociates to form water with the OH ⁻ in solution.

22 ESTER HYDROLYSIS Esters are hydrolysed using water in the presence of a catalyst (either an acid or an alkali). Dilute sulphuric acid is used with acid hydrolysis, the excess water shifting the equilibrium to the right and producing more product (carboxylic acid). Alkaline hydrolysis (i.e. via NaOH) doesn’t produce a carboxylic acid, but a carboxylate salt (acid dissociates in alkaline conditions). This hydrolysis goes to completion and so is preferred.

23 CONDENSATION OF THE NH ₂ GROUP The reaction of an amine group and a carboxylic acid or acyl chloride produces a polyamide, also called a nylon. In this condensation reaction, the functional groups react to form a peptide linkage (in proteins, amino acid monomers) or secondary amide link (non-proteins) and also a molecule of water or HCl depending upon which out of -COOH and -COCl were used. The process is similar to that of the reaction between diols and dicarboxylic acids to form polyesters and in fact we can simply substitute the diol for a diamine to get our polyamide. In industry this reaction happens slowly, but can be sped up in the lab with the presence of an acyl chloride. NH ₂ -- NH ₂ HOOC -- COOH NH ₂ -- NH ₂ HOOC -- COOH HOOC -N -- C - = H -- O - NH ₂ N -- C - = H -- O - N- C - = H -- O +H ₂O

24 PURIFICATION TECHNIQUES CHEMICAL IDEAS APPENDIX B5 START -- PURIFICATION TECHNIQUES

25 RECRYSTALLISATION Recrystallisation is used to obtain a pure solid from an impure, crude solution or solid. 1.Dissolve the solid in a minimum amount of suitable solvent. The solvent will be one in which the desired solid is soluble at high temperatures but not a t low temperatures. 2.Heat the solution, dissolving the desired product and causing any impure solids that do not dissolve in the chosen solvent, to leave the solution and form a solid. 3.Whilst still hot, and ideally with pre-warmed apparatus, filter the solution into a conical flask and retain the filtrate. The solid is impure waste. 4.Leave the filtrate to cool, allowing the pure solid to recrystallise out of solution. 5.Use vacuum filtration to remove unwanted liquid and leave behind crystals. 6.Wash and dry the crystals.

26 GREEN CHEMISTRY CHEMICAL IDEAS 15.10 START -- GREEN CHEMISRY

27 RECYCLING POLYMERS Recycling polymers has many benefits, including; reducing the amount of solids waste sent to landfill sites, reducing emissions of carbon dioxide, nitrogen (II) oxide and sulphur dioxide, reducing fossil fuel consumption and energy consumption. Waste polymers, such as those from industry that are simply leftover or unwanted, are named “process scrap” and are simple and economical to “reprocess”. After the polymers/plastics have been used however, man power and time make sorting through various grades of plastic a time consuming and costly process. After sorting however, the plastics are simple melted down and remoulded or are granulated and processed later. Breaking down the plastics into their constituent monomers is called chemical/feedstock recycling and can be used to regenerate monomers that can then be used again in petrochemical and chemical production. Manufacturers can also look into the use of bioplastics to improve their adherence to green chemistry. This focuses on the use of renewable sources of polymer such as plants and genetically modified plants and organisms. The fact that manufacturers are already dealing with some form of the finished product, the energy required to recycle polymers is much less than that required to make them from their original, raw materials. In this way, recycling is economical, saves on feedstocks, fossil fuels and carbon emisssions.


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