Weava Collection - Research on Chemistry (bonds, carbon, acids, molecules, water)
Organic chemistry: 10.32 - Alkenes
- Changing double bonds to single bonds, via hydrogenation, increases the melting point of a vegetable oil, an essential step in the production of margarine.
- Addition to asymmetric alkenes
Asymmetric alkenes have different groups on either side of the double bond. For example propene, CH2CHCH3.
When electrophilic addition occurs with an asymmetric alkene there is a choice of possible products.
In practise both of the possible products are formed, but in different amounts. There is a major product with a higher percentage yield and a minor product with a lower percentage yield.
Markovnikov's rule can be applied to predict the major product.
Markovnikov's rule says that the hydrogen atom from the electrophile (HBr in this case) preferentially adds to the carbon atom of the double bond that already has the most hydrogen atoms attached.
It may be seen that in the above example the major product is 2-bromopropane.
The major and minor products can be explained in terms of the relative stability of the two possible carbo-cation intermediates formed in the reaction mechanism.
- The longest chain with the double bond has three carbon atoms. The chain is numbered to keep the double bond with the lowest number. The bromine, therefore is on carbon number 3. Hence 3-bromopropene
- Mechanism - electrophilic addition
- Alkenes can form addition products with other molecules by 'opening' the double bond and using the electrons to form bonds at each carbon atom. This is known as addition.
The process is stimulated by electrophilic attack by the reagent, and for this reason is called electrophilic addition.
Organic chemistry: 10.12 - Formula representation
- The structural formula shows the actual arrangement of the atoms in a molecule by drawing the bonds as lines between letters representing the atoms.
- In a skeletal formula each carbon is represented by an angle, or termination in a line and the hydrogen atoms are just assumed
- The condensed formula is a shorthand method of representing the structural formula which relies on some knowledge of chemical structures.
- The molecular formula shows the actual number of atoms of each type in the molecule.
- The empirical formula is the simplest ratio of the atoms within a molecule of the compound.
Organic chemistry: 10.35 - Carboxylic acids
- The lower members of the homologous series are high boiling point liquids. This is due to the high degree of hydrogen bonding that holds the carboxylic acid molecules together. There is evidence that the acids exist as dimers even in the vapour state.
High relative molecular mass carboxylic acids are solids for the same reasons, i.e. strong intermolecular forces.
- Carboxylic acids can be reduced to aldehydes, and further to primary alcohols, by strong reducing agent such as lithium aluminium hydride (lithium tetrahydroaluminate). This reagent is used in ethoxyethane (diethyl ether) as it is very water sensitive. The complex formed is then hydrolysed by dilute acid to the final product.
CH3COOH + [LiAlH4] CH3CHO
CH3CHO + [LiAlH4] CH3CH2OH
- Carboxylic acids are weak acids. They undergo reactions typical of acids, they react with alcohols to form esters, the OH group can undergo substitution reactions with powerful reagents and the carboxyl group can be reduced by powerful reducing agents such as lithium aluminium hydride (lithium tetrahydroaluminate) to aldehydes and, subsequently, primary alcohols.
- Esterification is the reaction between a carboxylic acid and an alcohol forming an ester and water.
CH3COOH + CH3OH
CH3COOCH3 + H2O
The reaction is an equilibrium and it is helped by the addition of concentrated sulfuric acid which both acts as a catalyst, speeding up the attainment of equilibrium, as well as a dehydrating agent, mopping up the water as it is formed and pulling the equilibrium to the right hand side (according to Le Chatelier's principle).
Naming the product
The original carboxylic acid provides the ending of the name carboxylate
The original alcohol gives the beginning of the ester's name alkyl
- The strength of a carboxylic acid depends on two factors:
The ease with which the O-H bond is broken
The relative stability of the ion formed
The first factor is affected by electron withdrawing groups attached to the carbon chain. These draw electron density away from the carboxylic acid group, weakening the O-H bond, making loss of the hydrogen ion easier. The consequence is greater acidity. Electron inducing (pushing) groups such as the alkyl group have the reverse effect.
The acid strength is given by the acid equilibrium constant, Ka, or its pKa value. For the equilibrium:
H+(aq) + CH3COO-(aq)
The acid dissociation constant is given by:
The stronger an acid is the greater its Ka value and the smaller its pKa value.
- The -COOH functional group dissociates in aqueous solution according to the equation:
H+(aq) + CH3COO-(aq)
This means that carboxylic acids undergo all of the usual reactions of acids, if a little more slowly.
With bases making salt and water
With carbonates making salt + water + carbon dioxide
With active metals making salt + hydrogen
The acidic nature of carboxylic acids can be used as a test. The suspected carboxylic acid is added to a solution of sodium hydrogen carbonate and if bubbles of gas are seen then this is taken as a positive identification.
Organic chemistry: 10.31 - Alkanes
Organic chemistry: 10.18 - Optical Isomerism - HL
- Optical isomers have an identical arrangement of atoms, but the isomers are mirror images of one another. These mirror images are non-superimposible, just like a pair of gloves. This can only happen if the molecule is asymmetrical. The easiest way for a molecule to be asymmetrical is to have a carbon atom with four different groups or atoms attached. Such an asymmetric carbon atom is called a chiral atom, or centre of chirality.
Mirror image optical isomers are called enantiomers.
Physical properties of enantiomers
Chemical properties of enantiomers
Measuring optical rotation
Acids and bases: 8.53 - Acid and base dissociation constants
- Example: Hydrogen ion concentration from Ka.
Calculate the [H+(aq)] of 0.2 M ethanoic acid (Ka = 1.78 x 10-5)
Organic chemistry: 10.33 - Alcohols
- Alcohols react with carboxylic acids producing esters.
- Alcohols are oxidised by strong oxidising agents such as potassium dichromate (VI) in acidic solution. The products depend upon the nature of the alcohol.
Primary alcohols produce aldehydes
Secondary alcohols produce ketones
Tertiary alcohols are not easily oxidised (no reaction with potassium dichromate in acidic solution)
- Alcohols with two or more carbons can be dehydrated (the elements of water, H2O, removed) by heating at 170ºC with concentrated phosphoric acid, sulfuric acid or by passing over hot aluminium oxide.
- Alcohols, like the majority of organic compounds, burn in air or oxygen. The products of complete combustion are carbon dioxide and water. In a restricted supply of air the products will include carbon monoxide and possibly carbon as well as carbon dioxide and water.
- methanol + ethanoic acid methyl ethanoate + water
CH3OH + CH3COOH CH3COOCH3 + H2O
The reaction is catalysed by concentrated sulfuric acid, which also absorbs the water formed, pulling the equilibrium to the right hand side.
Organic chemistry: 10.22 - Solubility
- Homologous series
alkanes, alkenes Insoluble
halogenoalkanes, amines (more so than halolkanes) Slightly soluble
alcohols, carboxylic acids, aldehydes, ketones, amides Soluble
Increasing the length of the hydrocarbon chain reduces the solubility.
- Thermodynamically for a substance to be soluble the Gibbs Free Energy change for the process must be negative. Gibbs Free Energy change is related to the enthalpy and entropy changes at a specific temperature by the equation:
Clearly, the temperature at which the solubility is measured has a bearing on the result. Most substances increase solubility as the temperature is increased.
Two factors that make prediction of solubility a little easier in organic chemistry are:
Hydrocarbon chains are non-polar and do not form bonds with water molecules. They are said to be hydrophobic (water hating).
Some functional groups form hydrogen bonds with water, allowing the molecule to dissolve. These groups are said to be hydrophilic (water loving)
Acids and bases: 8.13 - The properties of bases
- Alkali is simply the term given to a soluble base, such as sodium hydroxide, potassium hydroxide or barium hydroxide.
Acids and bases: 8.22 - Lewis theory
- All transition metals form coordinate bonds with ligands. This means that they accept electron pairs from the ligands. They behave as Lewis acids. The reacting ligands are Lewis bases.
Redox processes: 9.31 - Relative reactivity
- Metals react by losing electrons - they are reducing agents. Non-metals react by gaining electrons - they are oxidising agents.
Organic chemistry: 10.21 - Volatility
- A chlorine atom, for example, has an electronegativity of 3.0. Consequently chlorine attracts electrons when bonded to carbon and develops a partial negative charge, leaving a partial positive charge on the carbon. This is called a permanent dipole. These permanent dipoles attract a molecule to neighbouring molecules.
- It happens when a molecule contains hydrogen attached to oxygen or nitrogen. These two elements are highly electronegative and draw the electrons away from the hydrogen atom. As hydrogen only has one electron in the first place, the effect exposes the hydrogen nucleus, causing a high partial charge density. This makes the N-H, or O-H bond, very polar and, in consequence, dipole-dipole forces are very strong (about one tenth of a normal covalent bond).
- All covalent molecules have these forces. They are caused by vibrations within the molecule that produce temporary dipoles. These temporary dioles vibrate sympathetically in neighbouring molecules leaving the partial positive part of the dipole next to the partial negative side of the dipole and vice-versa.
dispersion forces act between ALL molecules regardless of their structure. The strength of the force depends on two structural features:
the relative molecular mass of the compound
the shape of the compound
- Volatile means to be turned into a vapour easily. Liquids turn into vapour at all temperatures, because, according to the Maxwell-Boltzmann energy distribution curve, there are always some particles with enough energy to escape the body of liquid. The more volatile a substance is the easier is the vaporisation process
- The volatility of a covalent liquid depends on the strength of the intermolecular forces, as these must be overcome in order for the particles to change from liquid state to gaseous state.
Equilibrium: 7.27 - Industrial processes
- The contact process
- The Haber process
Organic chemistry: 10.15 - Hydrocarbons
- Aliphatic hydrocarbons - straight and branched molecules
Alicyclic hydrocarbons - ring structures
Aromatic hydrocarbons - benzene and other delocalised ring structures
- All hydrocarbons are flammable and produce carbon dioxide and water when burned in excess air or oxygen. Incomplete combustion may produce carbon monoxide and carbon microparticulates (small carbon containing particles). These products of incomplete combustion are a pollution hazard.
All complete combustion reactions can be represented by a general equation:
- When a molecule contains many double (or triple) bonds it is said to be polyunsaturated. In long chain hydrocarbons unsaturation gives rise to 'kinks' in the structure and prevents the long chains aligning easily. This lowers the melting point.
Organic chemistry: 10.34 - Halogenoalkanes
- Cannot donate hydrogen ions to the solute. They can however be either polar or non-polar.
- Summary of nucleophilic substitution in haloalkanes
- The rate of reaction is dependent on two factors:
Whether the haloalkane is primary, secondary of tertiary
The actual halogen atoms attached to the alkyl chain
Effect of mechanism
Tertiary haloakanes react via an sN1 mechanism that has a much lower activation energy than the sN2 mechanism with the high energy transition state. Hence tertiary haloalkanes react faster then secondary, which in turn react faster than primary.
Effect of the halide ion
The bond fromed between carbon and iodine is much weaker than the bond formed between bromine and carbon, which in turn is weaker than the bond formed between chlorine and carbon. It is therefore easier to break off an iodide ion than a bromide or chloride ion. The rate of reaction is in the order:
- Halogen atoms are electronegative, and as such they pull or draw electron density towards themselves, and away from any carbon to which they are attached, creating polarised bonds. This leaves a partial positive charge on the carbon atom, making it a reactive center.
- Mechanisms that proceed via an sN2 mechanism cause inversion at the carbon atom that is attacked by the nucleophile.
This has consequences for the optical properties and the designation of the configuration (R or S) of the stereoisomer.
If the enantiomer has an "R" conformation before nucleophilic substitution, as determined by CIP priority rules, then the product of the substitution has an "S" configuration
- Protic solvents have a labile (easily released) hydrogen ion. These are usually hydrogen atoms attached to either oxygen or nitrogen.
Equilibrium: 7.23 - The equilibrium law
Organic chemistry: 10.16 - Isomerism
- Isomerism means molecules that have the same molecular formula, but differ in the arrangement of their atoms relative to one another, either structurally or spatially.
Organic chemistry: 10.17 - Cis-trans and E/Z isomerism
- Isomers are named by using the prefix 'cis' to indicate that important groups are on the same side of the chain, or 'trans' to indicate that the important groups or atoms lie on opposite sides of the carbon chain.
- When the highest priority groups are on the same side of the double bond the isomer is assigned the letter "Z", which comes from the German "Zusammen" meaning together. An easy way to remember is that Z isomers have the highest priority groups on the Zame Zide.
When the highest priority groups are on opposite sides of the double bond the isomer is assigned the letter "E" from the German "Entgegen" meaning against or towards.
- The most important group of compounds that exhibit geometric isomerism is the alkenes. There is no rotation about a double bond due to the nature of the 'pi' bond system, which would break if the molecule were to rotate about the carbon - carbon bond.
- This is when two molecules have the same molecular formula and the same groups of atoms, but they are arranged in such a way that different forms of the molecule are non-superimposible.
There are two types of stereoisomerism:
Cis-trans or E/Z isomerism
- The relationship between the atoms within each structure is unique, giving rise to different chemical and physical properties. This is true for all geometric isomers.
Organic chemistry: 10.13 - Homologous series
- The solubility in water depends on the ability of the water molecules to attract dipoles in the organic compound. Functional groups may contain dipoles that make the organic molecule soluble to a lesser, or greater extent. These dipoles are said to be hydrophilic (water loving). However, the carbon chain itself has no dipoles and is not attractive to water. It is said to be hydrophobic (water hating).
The difference between each member of an homologous series is one -CH2- unit. The CH2 group is non-polar (hydrophobic) and this increases the percentage of the molecule that is unattractive to water. Organic compounds consequently tend to be less soluble as an homologous series is ascended.
- A general formula is one in which the numbers of carbons are represented by the letter 'n' and the numbers of other elements as a function of 'n'
So if there are two carbons and four hydrogens, n is equal to 2 (the number of carbons) and the value for the hydrogens = 2n. The general formula becomes CnH2n.
- Both density and boiling point depend on the intermolecular forces in the compound. These may be due to Van der Waal's forces only, or a combination of Van der Waal's and dipole-dipole attractive forces. In either case, the Van der Waal's force increases as the relative molecular mass increases, causing a corresponding increase in density and boiling point.
Acids and bases: 8.71 - Salt hydrolysis
- When both ions come from strong acid and bases, they have no interactions with the ions formed by the dissociation of water (hydrogen and hydroxide ions), however if the ions come from weak acids and bases, then they interact with the ions from water establishing equilibria.
Organic chemistry: 10.11 - Organic structure
- Those compounds that contain carbon and hydrogen only are called hydrocarbons.
- The definition of organic compound is now taken to mean a compound of carbon that is not a simple mineral compound.
- Carbon single bonds are of the type sigma, caused by direct orbital overlap along a linear axis.
Carbon double bonds consist of one sigma bond and one 'pi' bond, caused by lateral (sideways) overlap of two parallel orbitals. Notice that the overlap happens above and below the sigma bond. These two overlaps constitute only one pi bond.
Triple bonds formed by carbon atoms comprise one 'sigma' and two 'pi' bonds. The 'pi' bonds are at right angles to one another. In a triple bond with two 'pi' bonds there are four regions of overlap corresponding to the two pi bonds and one region of overlap along the axis joining the two atoms corresponding to the sigma bond.
The geometry of a four bonded carbon is tetrahedral with bond angles of 109.5º. The geometry of carbon bonded to three other atoms is trigonal planar (bond angle 120º) The geometry of carbon bonded to two other atoms is linear (bond angle 180º)
Acids and bases: 8.12 - The properties of acids
- An amphoteric substance can neutralise both acids and bases. Such substances are exemplified by oxides of 'poor' metals, i.e. metals from the centre of the periodic table, such as aluminium oxide and zinc oxide.
Organic chemistry: 10.14 - Naming organic molecules
- Systematic nomenclature
Multiple attached groups
Primary, secondary and tertiary
Nitrogen containing compounds
Bridging oxygen atoms