Fundamentals of Organic Chemistry Flashcards
Unit 1, Lectures 1-8 - Heulyn Jones
Double bond equivalents
If a double bond were introduced to a molecule, the molecule would naturally lose 2 hydrogens - double bond equivalent of 1. Cyclic structures are double bond equivalents too - benzene has a DBE of 4, as it has 3 double bonds and 1 ring.
DBE formula
(2n + 2) - (no. H)/2
DBE: nitrogen
Nitrogen acts as an exception, as for every N added, a H is also added. The formula is thus changed to account for this: (2n + 3) - (no. H)/2. (for 1 N)
What can a substitution on the 2nd carbon also be known as? (e.g. propan-2-ol)
Isopropanol
Oxidation state
A number assigned to an element in a chemical combination that represents the number of electrons lost (or gained, if the number is negative) by an atom of that element in the compound.
Oxidation level
The number of bonds a carbon has to a non-C/H atom. (C=O counts as two C-O single bonds.)
Formyl group
Carboxylic acid group
Acetyl group
Carbonyl group
Acetate group
Ester group
Bond angles/length: C-C
109, 1.57
Bond angles/length: C=C
120, 1.35
Bond angles/length: C≡C
180, 1.21
What bonds do sp3 hybridised orbitals form?
Sigma bonds
What bonds do sp2 hybridised orbitals form?
Sigma and pi bonds - e.g., ethylene: sigma bond formed from the head-to-head interaction between 2 sp2 hybridised orbitals and pi bond formed from the sideways overlap of the two extra p orbitals. Pi bond is substantially weaker.
What bonds do sp hybridised orbitals form?
C-C triple bonds - as each carbon has two unused p orbitals, it can form 1 sigma bond and two pi bonds
Hybridisation in nitrogen
NH3 has a bond angle of 107.3, which suggests sp3 hybridisation. As N has one more electrons than C, one of the sp3 orbitals is fully occupied - lone pair. Ths lone pair ensures the terahedral shape of NH3.
Newman projection
A conformation that shows a molecule from sideways on, showing how the attached groups are orientated and can display the optimum orientation based on energy/stability.
Anti Newman conformation
Most stable projections are achieved when non-hydrogen groups are placed as far apart from each other as possible.
Eclipsed Newman conformation
Least stable conformation when non-hydrogen groups are placed adjacent to each other
Chair conformations
An orientation of cyclid compounds that displays them “lying”down”—helps to show most stable orientation using equatorial and axial positionings.
Most stable chair conformations
Placing a group larger than hydrogen equatorially is always more stable. The larger the group, the bigger preference for an equatorial conformation.
Chair conformations: more than one substituent
May have to choose which substituent is more likely to be placed equatorially. The bigger group would sit equatorially.
Constitutional isomer
Same empirical formula, atoms are connected differently.
Configurational isomer
A subtype of stereoisomerism where centres aren’t chial, have the same empirical formula and connectivity, but are not identical 3D shapes. Cis (z) or trans (e) isomers
How is priority assigned in stereochemistry
Atomic weight
Enantiomers
Non-superimposable mirror images
S enantiomer
Anti-clockwise
R enantiomer
Clockwise
Properties of enantiomers
Have the same mp and bp
Have the same solubility
Produce the same IR and NMR spectra
Polarise light
Levorotatory
Enantiomers that polarise light to the left (L/-)
Dextrorotatory
Enantiomers that polarise light to the right (R/+)
For each stereogenic centre, how many possible stereoisomers are there?
2
Stereoisomers: exception
Meso compounds: have 2 stereogenic centres but only 3 stereoisomers.
Racemic mixtures
50:50 mixture of S and R enantiomers.
Polarised light will not be rotated as the enantimers cancel one another out.
Separating enantiomers
Gas/liquid chromatography using a chiral separating column - relative integrations give measure of enantiomeric mixture.
NMR analysis of enantiomers
As the NMR spectra would be identical, must form diastereomer first.
Diastereomer
stereoisomers that are not mirror images
Conjugated alkenes
Alkenes that have alternating double bonds. More energetically stable.
Mesomeric effect
How electron density is distributed in a molecule due to delocalisation of electrons. Occurs when there are double bonds or lone pairs that can share electrons, thus changing its reactivity and stability.
Conjugate nucleophilic addition
Where a nucleophile attacks the second C away from a carbonyl bond rather than the carbonyl C, as the carbonyl bond pulls electron density towards itself, making the beta carbon electron deficient and an easy target for nucleophiles.
Criteria for aromaticity
Molecule must be cyclic
Molecule must be flat
Pi-electrons must be conjugated
No of pi electrons must = 4n + 2 (n being the number of aromatic rings)
How to know if a cyclic structure is flat
If all carbons are sp2 hybridised, they are all in the same plane.
Ortho position
2nd carbon in a ring
Meta position
3rd carbon in a ring
Para position
4th carbon in a ring
Aromatic stability
High stability, low energy due to electrons being delocalised across the pi system.
Why are aromatic molecules more reactive towards electrophiles?
The electron-rich pi system attracts electrophiles as they are electron-deficient
Why are carbocations sp2 hybridised?
In the sp2 hybridised state, C has 3 bonds (2 single and 1 empty p orbital). As the positive charge is found in the empty orbital, it can interact with adjacent bonds and spread out the positive charge, neutralising it and thus stabilising the carbocation.
Geometry of carbocations
Trigonal planar, 120
Hyperconjugation -carbocations
The process by which nearby sigma orbitals of CH bonds can donate electron density into the empty p-orbital of a carbocation. The more nearby CH bonds, the greater the degree of stabilisation.
Stabilising effects of heteroatoms
If the nearby bonds are electron-donating (resonance or induction), such as OH, ether groups, and amines, the carbocations tend to be more stable overall, as the positive charge in the empty p orbital is more neutralised. With electron-withdrawing bonds, e.g., carbonyls or halogens (Cl, Br, I), the carbocation becomes more electron-deficient, however it may not destabilise if they are close enough to stabilise the carbocation through resonance.
Resonance stabilisation
Heteroatoms with lone pairs can delocalise positive charge via resonance, donating electron density, meaning that the positive charge is spread over a larger area.
Inductive effect
Electron withdrawing groups can also stabilise the carbocation by withdrawing electron denisty through the sigma bond. Though the carbocation becomes more electron deficient, the positive charge becomes more spread out and is neutralised that way, rather than adding electron density.
Hyperconjugation vs resonance vs induction
Resonance is typically the most effective at stabilising a carbocation, as it utilises pi bonds and lone pairs to delocalise charge over multiple atoms, spreading electron density further.
Hyperconjugation is also effective, but for alkyl carbocations in particular, as it relies on an abundance of adjacent CH and CC bonds from which to draw electron density.
The inductive effect is generally less effective, as it only works through sigma bonds, and its efficacy diminishes with increased distance. It also doesn’t involve the delocalisation of charge, but rather just donates or withdraws electrons.
Carbocations
An ion with a positively charged carbon atom.
Carbanions
Ions with a negatively charged carbon atom - generally more stable than carbocations.
Why are carbanions sp3 hybridised?
As they have an extra electron, which gives a negative charge. This charge occupies an empty p orbital, which can form bonds through overlap with other orbitals
Carbanion molecular geometry
Tetrahedral, 109.5
Organolithium reagents and grignard reagents
e.g., RLi, RMgX—used as substitutes for pure carbanions as they are too reactive (despite being more stable than carbocations.) They have a polarised bond where the carbon has a partial negative charge, allowing them to react more stably like carbanions. More substituted compounds tend to be less stable as resonance is not available from nearby groups with partial charges.
pKa
Used to scale the acidity of different protons. Strong acids will give high Ka and low pKa, and vice versa.
How does hybridisation affect acidity?
Having a higher s character in a compound stabilises negative charge as the neg charge is closer to the nucleus. Sp3 carbons are less acidic as they have 25% s character, sp2 have 33%, and sp have 50%.
Effects of conjugation and aromaticity on acidity
Higher acidity as the conjugated double bonds spread out negative charge and make it easier for the molecule to give up the hydrogen. Aromatic compounds have even more delocalised electron systems and therefore are more acidic.
Why are esters less acidic than ketones?
The conjugation in the carbonyls is weaker in esters than in ketones (due to the other oxygen), so they have a less effective delocalising effect. The lone pairs in esters can participate in resonance, but it’s not as effective as in ketones.
Prioritising resonance structures
More stable resonance structures have atome that obey the octet rule.
As few formal charges as possible (diff between charge of an atom and the charge it has in a molecule).
Negative charges on highly electronegative atoms.
Positive charge on less electronegative atoms.
Aromaticity increases stability.
