Advanced Higher Chemistry - Organic

Describe the molecular formula.

It doesn't show functional groups,

Shows the number of each type of atom.

e.g C2H2O2, C5H10

How can an empirical formula be obtained?

Elemental microanalysis

What does the empiras formula show?

The simplest ratio of atoms.

e.g CH3 (C2H6)

What does the full structural formula show?

All atoms and bonds present

What does the shortened structural formula show?

All atoms and groups of atoms,

It doesn't show every bond

e.g CH3CH2CH3

What does the skeletal formula show?

It doesn't show the C or H, each "kink" represents a carbon and it is assumed to have the right number of hydrogens

What is stereoisomerism?

A type of isomerism in which molecules have the same molecular formula and atoms are bonded together in some order but arrangement of atoms is different

What are the 2 types of stereoisomerism?

Geometric and optical isomerism

Where does geometric isomerism occur?

In molecules containing C\=C or ring structure as there is no free rotation of groups of atoms around the double bond

What are the 2 types of geometric isomerism?

Cis and trans

What does cis mean?

The substituent groups are on the same side

What does trans mean?

The groups are on opposite sides

Which is more stable, cis or trans alkenes?

Trans

Why are trans alkenes more stable than cis?

Due to steric hindrance of 2 groups on the same side of C\=C.

Are cis and trans the same compounds? And what does this mean for their physical and chemical properties?

They are different compounds so have different physical and chemical properties

Why do trans isomers have a higher melting point and higher density?

They have a straighter molecule shape so can pack closer together. This leads to a higher density and stronger intermolecular forces between the molecules, resulting in more energy being required to overcome these forces

Are trans fats common in nature?

No

How are artificial trans fats formed?

When unsaturated oils undergo addition reactions with hydrogen to make them less unsaturated and more solid

What are the downsides to producing inactive drugs?

They waste materials, money and need higher dosage

Where does optical isomerism occur?

In chiral molecules where 4 different groups are arranged around a central carbon atom known as a chiral atom

What are optical isomers?

Non-superimposable mirror images of each other

How are optical isomers similar/ different form each other?

They have identical physical and chemical properties but different optical activities

How are optical activities measured?

Using a polarimeter

What is plane-polarised light?

Light travelling in only 1 plane

What do optical isomers do to plane polarised light?

Rotate plane polarise light through same angles but different directions

What does a clockwise rotation of plane polarised light mean?

It is dextrorotatory, the + isomer

What does an anti-clockwise rotation of plane polarised light mean?

It is levorotatory, the - isomer

What is a racemic mixture?

When there are equal quantities of the 2 optical isomers present

Do racemic mixtures have an effect on polarised light and why?

They do not since they are optically active

How does elemental microanalysis work?

A known mass of a compound is burned in excess oxygen to produce CO2, H2O, SO2, NOx. The mixture of gas is carried in a stream of helium over heated copper which reduces NOx to N. The mass of the remaining compounds can then be calculated

What information does mass spectroscopy give?

The molecular mass of a compound and structural fragments present in the compound

What is m/z ratio?

The mass to charge ratio of the fragment

What will the peak with the highest m/z ratio represent?

The un-fragemented parent molecule (which gives the molecular mass of the compound)

What is Infra-Red spectroscopy used for?

To identify certain functional groups in organic compounds

What does IR spectroscopy do to bonds?

Causes them to vibrate, stretch and bend.

What does the wavelength which affects the bonds depend on?

The type of bond present and atoms in these bonds

What does a wide peak suggest?

Hydrogen bonding

What is +H NMR spectroscopy?

Proton nuclear magnetic resonance

What can +H NMR spectroscopy give information of?

It can give information about different environment of hydrogen atoms in a molecule and about how many hydrogen atoms are in each of these environments

What do protons posses and what does this mean for the nucleus of hydrogen atoms?

A property called spin so the nucleus of hydrogen atoms behave like tiny magnets

What do nuclei do in a magnetic field?

Align themselves with the field

What will nuclei parallel to the magnetic field have?

Lower energies than those that oppose

What can energy in the form of radio waves be used for?

To change the hydrogen nuclei within the magnetic field from the lower state to a higher state

What does the energy difference depend on?

The environment the nuclei is in (electrons shield the nuclei from the magnetic field)

What happens when the nuclei fall back to a lower state?

Energy is emitted and can be detected which results in peaks called chemical shifts

What does the frequency of emitted radio waves tell us?

It gives us information on position of Hydrogen atoms in the molecule

What does the area under the peak correlate to?

The number of Hydrogen atoms in that environment

What is TMS used for?

A standard reference substance when running NMR spectroscopy

What is the chemical shift value of TMS?

0

What are the 2 types of +H NMR spectrum?

Low resolution and high resolution

Describe low resolution.

The chemical shift and area under the peak is interpreted

Describe high resolution.

Interactions with Hydrogen atoms on neighbouring Carbon atoms can result in splitting of peaks into multiplets

How can the number of peaks in a multiplet be calculated?

n +1, where n is the number of hydrogen atoms on neighbouring carbon atoms

What type of orbitals do electrons involved in bonding occupy?

Bonding molecular orbitals

Desire anti-bonding molecular orbitals.

They re usually empty unless the number of electrons available for bonding would violate the Pauli principle

What is the order of energy with the orbitals?

bonding molecular orbitals < atomic orbitals , anti bonding orbital (most energy)

What is the hydride ion?

H^-

Are hybrid orbitals degenerate?

Yes

What are π molecular orbitals/ π 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

How do alkenes hybridise?

Their 2s orbitals and 3 2p orbitals of carbon hybridise to form 4 degenerate sp3 hybrid orbitals, which adopt a tetrahedral arrangement

How do the sp3 orbitals form σ bonds?

They overlap end-on with other atomic orbitals

Is the C-C in alkanes able to rotate freely?

Yes

What is the type of hybridisation in alkenes?

The 2s orbitals and 2 2p orbital hybridise to form 3 degenerate sp2 hybrid orbitals which adopt a trigonal planar arrangement

How do the sp2 orbitals form σ bonds?

They overlap end-on

What happens to the remaining unhybridised 2p orbital in alkenes?

It lies perpendicular to the axis of σ bond and they overlap side one to form π bonds

What does a C\=C consist of? (in terms of pi and sigma bonds)

1 σ + 1π bond

What is the type of hybridisation in benzene and other aromatic compounds?

sp2

How are benzene rings arranged?

6 carbon atoms are arranged cyclicly with σ bonds between carbon atoms

What do the unhybridised p orbitals do in benzene rings?

The overlap to form a π molecular system, perpendicular to the plane of π bonds

What is the type of hybridisation in alkynes?

2s and 2p orbitals of carbon hybridise to form 2 sp degenerate hybrid orbitals, adopting a linear arrangement

What happens to the unhybridised p orbitals in alkynes?

They overlap side on to form 2π bonds

What do the 2 hybrid sp orbitals in alkynes do?

Overlap end-on to form σ bonds

What does a C-C consist of? (in terms of π and σ bonds)

1 σ bond

What does a C≡C consist of? (in terms of π and σ bonds)

1 σ, 2 π bonds

What its energy form light used to do?

Promote electrons form bonding or non-bonding orbitals into the higher energy anti-bonding orbitals

What do σ and π orbitals contain?

Normal bonding pairs of electrons

What do non-bonding orbitals contain?

Lone pairs of electrons

What do σ* and π* anti-bonding orbitals contain?

They are normally empty

How do electrons fill the orbitals?

They fill bonding molecular orbitals, leaving the anti-bonding orbitals unfilled

What is the Highest Occupied Molecular Orbital (HOMO)?

The highest bonding molecular orbital containing electrons

What is the Lowest Unoccupied Molecular Orbital (LUMO)?

The lowest anti-bonding molecular orbital

What can cause electrons to be promoted from HOMO to LUMO?

Absorption of electromagnetic energy

Why do most organic molecules appear colourless?

Because the energy difference between HOMO and LUMO is relatively large which results in the absorption of light from the ultraviolet region of the electromagnetic spectrum

Why are some organic molecules coloured?

Dueto a conjugated system

What does a conjugated system contain?

Delocalised electrons spread over a number of atoms (alternating σ and π bonds)

Are benzene rings conjugated and why/ why not?

Yes because of the delocalised electrons

What are chromophores?

A group of atoms within a molecule that is responsive for absorption of light in the visible region the spectrum

When can light be absorbed?

When electrons in a chromophore are promoted from the HOMO to the LUMO

Where do chromophores exist?

In molecules containing a conjugated system

What is a conjugated system?

A system of adjacent unhybridised p orbitals that overlap side-on to form a molecular orbital across a number of carbon atoms

What does more atoms in the conjugated system mean?

A smaller gap between HOMO and LUMO

When frequency of light decreases, what happens to wavelength and energy?

Wavelength gets longer, energy decreases

When will the compound exhibit colour?

When the wavelength of light absorbed is in the visible region, the complementary colour is exhibited

What does a single-headed curly arrow mean?

It shows movement of a single electron

What does a double-headed curly arrow mean?

It shows the movement of a pair of electrons

Where should the arrow start and finish?

It should start at the origin of electrons and finish at the destination of electrons

What does it mean if an arrow points to a space between 2 atoms?

It indicates that a coolant bond will be formed between them

Describe what happens to bonds during an organic reaction.

Bonds in the reactant molecules are broken (bond fission) and bonds in the product molecules are being made

What are the 2 types of bond fission?

Homolytic and heterolytic