Unit 3- Organic chemistry and instrumental analysis Flashcards
When do molecular orbitals form?
When atomic orbitals combine
What is the number of molecular orbitals formed equal to?
The number of atomic orbitals that combine (2 atomic orbitals combine to make 2 molecular orbitals)
What does the combination of two atomic orbitals result in?
The formation of a bonding molecular orbital and an antibonding orbital
Which of the two molecular orbitals (bonding and antibonding) encompasses both nuclei?
The bonding molecular orbital
In molecular orbital theory what is the basis of bonding between atoms?
The attraction of the positively charged nuclei and the negatively charged electrons
How many electrons can each molecular orbital hold?
Each molecular orbital can hold a maximum of 2 electrons
What are sigma bonds?
Molecular orbitals that form by end-on overlap of atomic orbitals along the axis of the covalent bond
What are pi bonds?
Molecular orbitals that form by side-on overlap of parallel atomic orbitals that lie perpendicular to the axis of the covalent bond
What is hybridisation?
The process of mixing atomic orbitals within an atom to generate a set of new atomic orbitals called hybrid orbitals
What can be said about the new hybrid orbitals that are formed through hybridisation?
They are degenerate
What kind of hybridisation takes place in alkanes?
The 2s orbital and the three 2p orbitals of carbon hybridise to form four degenerate sp^3 hybrid orbitals
What arrangement do the sp^3 hybrid orbitals in alkanes adopt?
A tetrahedral arrangement
What kind of hybridisation takes place in alkenes?
The 2s orbital and two of the 2p orbitals of carbon hybridise to form three degenerate sp^2 orbitals
What can the bonding in benzene and other aromatic systems be described in terms of?
sp^2 hybridisation
Out of pure covalent molecules, polar covalent molecules and ionic molecules which show the most and least symmetry?
Most= pure covalent Middle= polar covalent Least= ionic covalent
In a triple bond how many sigma and pi bonds are there?
1 sigma bond and 2 pi bonds
The electronic configuration of an isolated carbon atom cannot explain the number of bonds formed by carbon atoms in molecules. What can be used to explain the bonding and shape of molecules of carbon?
Hybridisation
In a double bond how many sigma and pi bonds are there?
1 sigma and 1 pi
What kind of bond is present in a single bond?
A sigma bond
What arrangement does sp^2 hybridisation in alkenes adopt?
A trigonal planar arrangement
What can the binding in benzene and other aromatic molecules be described in terms of ?
Sp^2 hybridisation
What can be said about the electrons in benzene?
They are delocalised
What arrangement do the sp hybrid orbitals in alkynes adopt?
A linear arrangement
What is the HOMO?
The highest occupied molecular orbital, this is the highest bonding molecular orbital containing electrons
What is the LUMO?
The lowest unoccupied molecular orbital, this is the lowest anti-bonding molecular orbital
What can the absorption of electromagnetic energy in organic molecules cause to happen?
Electrons can be promoted from HOMO to LUMO
Why are most organic molecules colourless?
Because the energy difference between the HOMO and the LUMO is relatively large. This results in either electrons not making the jump or light being absorbed from the UV region of the electromagnetic spectrum
What is a chromosphere?
A group of atoms within a molecule that is responsible for the absorption of light in the visible region of the spectrum
What does it mean if their are more atoms in a conjugated system?
The more atoms there are the lower the energy gap between the HOMO and the LUMO. This means a higher wavelength of light will be absorbed
What is the process of bond breaking also known as?
Bond fission
What are the two types of bond fission?
Homolytic and heterolytic
What does homolytic fission result in?
The formation of 2 neutral radicals
What does heterolytic fission result in?
The formation of 2 oppositely charged ions
Which of homolytic and heterolytic fission is more suitable for organic synthesis and why?
Heterolytic fission, as they tend to result in fewer and less complex products
In reactions involving bond fission, what are attacking groups classified as?
Either nucleophiles or electrophiles
What are nucleophiles?
- negatively charged ions or neutral molecules
- electron rich
- attracted towards atoms bearing a partial or full positive charge
- capable of donating an electron pair to form a new covalent bond
What are electrophiles?
- positively charged ions or neutral molecules
- electron deficient
- attracted towards atoms bearing a partial or full negative charge
- capable of accepting an electron pair to form a new covalent bond
What 2 types of reaction are most commonly associated with alkenes (double bonds)?
Addition and elimination
What is the difference between aldehydes and ketones?
Aldehydes have a double bonded oxygen at the end of the carbon chain whereas in ketones it is in the middle of the chain
What is hydration?
An addition reaction (breaking a double bond) involving water
What is a condensation reaction?
Two molecules combine to form another, usually with the loss of a small molecule like water
What is the opposite reaction if condensation?
Hydrolysis
What happens in a neutralisation reaction?
An acid and a base are combined to form a neutral substance
How can aldehydes be prepared?
From the oxidation of primary alcohols
Can aldehydes and ketones be further oxidised?
Ketones cannot but aldehydes can be further oxidised to form carboxylic acids
How can ketones be prepared?
From the oxidation of secondary alcohols
What is the opposite of an addition reaction?
An elimination reaction
When naming esters where do the two parts of the name come from?
The first part comes from the alcohol and the second part comes from the carboxylic acid
What are the two ways to synthesis a haloalkane?
- Addition reaction of an alkene and a hydrogen halide to form a monohaloalkane
- Addition reaction of an alkene and a halogen to form a dihaloalkane
What are the 3 nucleophilic substitution reactions a haloalkane can undergo?
- Haloalkane reacts with aqueous NaOH or KOH to form an alcohol
- Haloalkane reacts with ethanolic cyanide to form a nitrile
- Haloalkane reacts with an alcoholic alkoxide to form an ether
What can happen to nitriles after they are synthesised from a haloalkane?
They can be hydrolysed to form a carboxylic acid
What happens during the reaction between a haloalkane and ethanolic cyanide to form a nitrile?
The carbon chain length is increased by 1
How do you form an alcoholic alkoxide?
By reaction an alkaline metal with an alcohol
What can be said about the Sn1 mechanism for haloalkanes?
- takes a minimum of 2 steps
- only 1 species in the rate determining step
- occurs via a trigonal planar carbocation intermediate
- a racemix mixture is formed
- used for tertiary haloalkanes
What can be said about the Sn2 mechanism for haloalkanes?
- takes 1 step
- 2 species in the rate determining step
- used for primary and sometimes secondary haloalkanes
Why is a race mix mixture formed during the Sn1 haloalkane mechanism?
Because the nucleophile can attack from either side
What is the 1 elimination reaction that haloalkanes can undergo?
- haloalkane reacts with a strong base (NaOH or KOH) in ethanol to form an alkene
When does hydrogen bonding occur?
When hydrogen is covalently bonded to either fluorine, oxygen or nitrogen
What are 4 properties of alcohols?
- high melting and boiling points due to OH group
- high solubility (however longer chain alcohols are not soluble)
- highly flammable
- good solvents
What are the 3 ways to prepare alcohols?
- Nucleophilic substitution of a haloalkane with aqueous NaOH or KOH
- Reduction of either a aldehyde, ketone or carboxylic acid using LiAlH4
- Acid-catalysed hydration of an alkene
What are the 4 reactions an alcohol can undertake?
- Oxidation of primary alcohols to form aldehydes and then carboxylic acids or secondary alcohols to form ketones
- Dehydration of alcohols to form alkenes using either Al2O3, H2SO4 or H3PO4
- Condensation reaction to form an ester either using a carboxylic acid or a acid chloride
- Reaction between an alcohol and a reactive metal to form an alcoholic alkoxide which can then react with a monohaloalkane to form an ether
What are the 2 methods of forming an ester and which is the preferred method?
- Condensation reaction of an alcohol with a carboxylic acid using conc. H2SO4 as a catalyst
- Condensation reaction of an alcohol with a acid chloride
Method 2 with the acid chloride is preferred as it is faster, not reversible and does not require a catalyst
What are 3 properties of ethers?
- low boiling points
- good solvents
- small ethers are soluble in water, large ethers are not
What are the 2 ways in which a alkene can be prepared?
- The dehydration of alcohols using either Al2O3, conc. H2SO4 or conc. H3PO4 as a catalyst
- The base-induced elimination of hydrogen halides from monohaloalkanes
What are the 4 electrophiles addition reactions an alkene can undergo?
- The hyd
What are the 4 electrophiles addition reactions an alkene can undergo?
- Hydrogenation (adding H+) to produce alkanes using a catalyst
- Addition reaction of an alkene and a halogen to produce a dihaloalkane
- Addition reaction of a alkene and a hydrogen halos to form a monohaloalkane
- Acid-catalysed hydration (addition water) to form an alcohol
What are the 5 ways in which a carboxylic acid can be prepared?
- Oxidising a primary alcohol using a oxidising agent
- Oxidising a aldehyde using an oxidising agent
- Acid or alkali hydrolysis of esters
- Acid or alkali hydrolysis of amides
- Acid hydrolysis of nitriles
What are the 4 reactions a carboxylic acid can undergo?
- Neutralisation reaction with a base to produce a salt
- Condensation reaction with an alcohol to form an ester, using either conc. H2SO4 or conc. H3PO4 as a catalyst
- Reaction with an amine to form an amide using heat
- Reduction reaction to form primary alcohols using LiAlH4
What do primary and secondary amines have that tertiary ones don’t?
Hydrogen bonding (N-H)
What can be said about boiling points of amines?
Primary and secondary amines have higher boiling points than comparative tertiary amines due to their hydrogen bonding
What can be said about the solubility of amines?
All types of amines can hydrogen bond with water which makes them soluble, smaller chain amines are more soluble
What is the molecular formula of benzene?
C6H6
What is the stability of the benzene ring due to?
The delocalisation of electrons in the conjugated system
What is a phenyl group?
A benzene ring in which 1 hydrogen has been substituted by another group. It has the formula C6H5-
What kind of reaction can benzene take part in?
Electrophilic substitution
What are the 4 reactions benzene can undergo?
- Halogenation by reaction with a halogen using a catalyst
- Alkylation by reaction with a haloalkane using aluminium chloride as a catalyst
- Nitration by reaction with conc. sulphuric acid and conc. nitric acid
- Sulfonation by reaction with conc. sulphuric acid
What are isomers?
Molecules with the same molecular formula but different structural formulae
What is the difference between structural isomers and stereoisomers?
Structural isomers occur when atoms are bonded together in a different order in each isomer. Stereoisomers occur when the order of the bonding in the atoms is the same but the spatial arrangement of the atoms is different in each isomer
What are the 2 types of stereoisomers?
Geometric and optical
What can be said about melting and boiling points in cis and trans isomers?
Trans isomers will have higher melting and boiling points as the molecules are packed tighter together allowing more intermolecular forces to be formed
When do optical isomers occur?
In compounds in which four different groups are arranged tetrahedrally around a central carbon atom
What can the carbon atom in the centre of a optical isomer be referred to as?
The chiral carbon or the chiral centre
What are enantiomers?
A pair of isomers that are non-superimposable mirror images of each other
Optical isomers have identical physical properties apart from…?
Their effect on plane-polarised light
Optical isomers have identical chemical properties apart from…?
When in a chiral environment such as that found in biological systems
What do optical isomers do to plane-polarised light?
Rotate it by the same amount but in different directions and so are optically active
What is formed if both enantiomers are present in equal amounts?
A racemix mixture, this will be optically inactive as the rotational effect of the plane-polarised light will cancel out
What are drugs?
Substances that alter the biochemical processes in the body
What are medicines?
Drugs that have beneficial effects
What is usually added to a medicine other than the drug?
Fillers to add bulk or sweeteners to improve the taste
How do drugs generally work?
By binding to specific protein molecules, these can either be found on the cell surface (receptor) or can be enzyme specific molecules within the cell
What does an agonist do?
Mimics the natural compound and generates a similar response
What does an antagonist do?
Prevents the natural compound from binding and blocks the natural response from occurring
What can many drugs that act on enzymes be classified as?
Enzyme inhibitors
How do enzyme inhibitors work?
By binding to the active site of the enzyme and blocking the reaction normally catalysed there
What are the types of interactions that can form between a drug and a receptor binding site?
van der Waals forces and/or ionic bonds
What does the structural fragment of a drug molecule that allows it to form interactions with a receptor binding site or to an enzyme normally consists of?
Different functional groups correctly orientated with respect to each other
What is elemental microanalysis used to determine?
The masses of C, H, O, S and N in a sample of an organic compound
What does an empirical formula show?
The simplest ratio of the elements in a molecule
What can mass spectrometry be used to determine?
A accurate gram formula mass (gfm) and structural features of an organic compound
What happens in mass spectrometry?
- a small sample of an organic compound is bombarded by high energy electrons
- this removes electrons from the organic molecule generating positively charged molecular ions called parent ions
- these molecular ions then break into smaller positively charged ion fragments
- a mass spectrum is obtained showing a plot of the relative abundance of the ions detected against the mass-to-charge ratio
What is infrared spectroscopy used for?
Identifying certain functional groups in an organic compound
What happens in infrared spectroscopy?
Infrared radiation is absorbed by organic compounds, bonds within the molecule vibrate (stretch and bend). The wavelengths that are absorbed depend on the type of atoms that make up the bond and the strength of the bond
How does infrared spectroscopy work?
Infrared radiation is passed through a sample of the organic compound and then into a detector that measures the intensity of the transmitted radiation at different wavelengths. The absorbable of infrared radiation is measured in wavenumbers
What is wavenumber?
The reciprocal of wavelength, 1/wavelength. It is given in units cm-1
What can proton NMR spectroscopy give information on?
The different chemical environments of hydrogen atoms in an organic molecule, and how many hydrogen atoms are in each of these environments
How does proton NMR spectroscopy work?
H nuclei behave like tiny magnets in a strong magnetic field. Some align with the field (lower energy), whilst the rest align against it. Absorption of radiation in the in the radio frequency region of the EM spectrum causes the H nuclei to flip from the lower to higher energy alignment. As they fall back to the lower energy alignment the emitted radiation is detected and plotted in the spectrum.
What is the standard reference used in proton NMR?
Tetramethylsilane (TMS), it is assigned a chemical shift value equal to zero
What is the difference between high and low resolution proton NMR?
High-resolution proton NMR uses higher radio frequencies than those used in low-resolution proton NMR and also provides a more detailed spectra
