Fundamentals Flashcards
(22 cards)
What is a double bond equivalent and how do you calculate it?
A calculation that determines the total number of rings and/or π bonds in a molecule based on its molecular formula.
OR The number of H2 molecules that would have to be added to a molecule to convert all pi bonds to single bonds, and all rings to acyclic structures. DBE=((2n+2)-(no. of hydrogens))/2
(2n+3) if includes 1 nitrogen
What is the Newman projection?
A different representation of a molecule, looking down the bond we are interested in.
Why is a staggered conformation more stable?
There is a repulsion between the electrons of the CH bonds – a staggered conformation places them as far away from each other as is possible.
There is a degree of stabilisation by donating electron density from one CH sigma bonding orbital to the adjacent CH sigma* antibonding orbital.
what are the two factors that make the anti conformation the highest in energy?
Sterics – putting two groups next to each other takes energy due to unfavourable steric interactions
Electronic – torsional strain from electrostatic repulsion of the electrons involved in the bonds that are being eclipsed.
What is a constitutional isomer?
Isomers that have the same empirical formula but where the atoms are connected differently.
What is a configurational isomer?
Where the molecules are not chiral, have the same empirical formula and connectivity of atoms, but are not identical in 3-D shape. They can also be called diastereomers – these molecules have different chemical and physical properties to their corresponding isomers.
What are enantiomers? How do you name enantiomers? R or S?
Enatiomers are non-superimposable mirror images. Chiral.
1. assign priority groups (atomic mass)
2. place the lowest priority group at the back (usually H, with wedges)
3. draw arrows from each priority group 1->2->3.
clockwise = (R) enantiomer
anti-clockwise = (S) enantiomer
How do you calculate the number of isomers a molecule with multiple stereogenic centres has?
2(to the power of stereogenic centres)
What are diastereomers?
Diastereomers are stereoisomers (molecules with the same chemical formula and connectivity but different spatial arrangements) that are not mirror images of each other, and typically occur in molecules with two or more chiral centers.
What is a meso compound?
A meso compound is an achiral (optically inactive) molecule that contains two or more chiral centers, but possesses an internal plane of symmetry that makes it superimposable on its mirror image.
What is a racemic mixture?
A 50:50 mixture of (S) and (R) enantiomers
How would you separate enantiomers? (chrial resolution)
React the mixture with a chrial acid, diastereomer salts form (soluble), then recrystallisation.
Analysis with nmr by converting enantiomers into diastereomers by reacting wih chiral shift reagents.
Gas/liquid chromatographyusing chiral stationary phase (columns)
What can conjugation lead to?
Extra bonding so extra stability due to better orbital overlaps and extended pi system.
What are the 4 conditions for aromaticity?
- the molecule must be cyclic
- the molecule must be flat
- the molecule’s pi-electrons must be conjugated
- the number of pi-electrons must be equal to 4n+2, this is called Huckel’s rule
What is hyperconjugation?
Nearby sigma orbitals of CH bonds can donate (delocalise) electron density into the empty p-orbital of the carbocation. It is a stabilising effect which leads to more substituted carbocations being more stable.
As the degree of unsaturation increases, so does the acidity/stability of the anion. Why is this?
hybridisation
Hybridisation. – alkanes are sp3 hybridised, alkenes are sp2 hybridised and alkynes are sp hybridised. The greater the s character of the hybridised orbital, the closer the electron density is held to the nucleus (the s-orbital by its very nature/shape is held closer relative to a p-orbital), which stabilises a negative charge.
What is the inductive effect?
Electronegative elements stabilise the negative charge of conjugate bases by pulling electrons (from the O-) through sigma bonds (making it a weak base, therefore making them strong acids)
– e.g. chlorine is an electronegative element that can help stabilise a negative charge.
How can carbocation stability be increased?
by conjugation, substitution, or degrees of resonance
What can a higher chemiscal shift show?
A higher chemical shift means the protons are higher de-shielded and to the left.
Protons with less electron density around them will have higher chemical shift. Having nearby electronegative groups nearby and having conjugation to take away electron density can increase chemical
What is spin-spin coupling?
Nuclei in different environments can interact with each other, either protons with other protons or with other spin-active nuclei.
What information can you gain from NMR?
- integration
- chemical shift
- coupling
How do you predict the multiplicity (number of peaks) of a signal based on the number of neighboring spin-active nuclei (n) and their spin quantum number (ms)?
2nI+1
where n is the number of equivalent, spin-active nuclei (like protons) neighboring the nucleus of interest, and I is the spin quantum number of the neighbouring nuclei.
