Introduction à la chimie organique Flashcards
What are the types of structural formulae?
- Condensed structural formula
- Displayed formula (full structural formula)
- Skeletal formula
Organic compound can b classified into what 2 main grp in terms of their carbon skeleton? Describe both
- Aliphatic/Alicyclic (non-aromatic)
Aliphatic -> C atoms joined in open chains (straight or branched)
Alicyclic -> C atoms joined in non-aromatic rings (alicyclic ring) - Aromatic
contains benzene
aromatic hydrocarbon
Organic compounds oso can b classified into what 2 main grp in terms of degree of saturation?
- Saturated (oni single bond)
- Unsaturated (hv multiple bonds eg double bond)
Define functional grp
an atom/grp of atoms common to series of organic compound that effectively determine principal chemical properties (eg type of rxn) of series
Define homologous series
series of compounds w same functional grp & general formula
What is an R- grp?
usually denotes alkyl grp, a grp derived fr alkane by removal of H atom -> general formula of R grp is -CnH 2n+1
eg methane -> methyl
What to take note of when naming organic compounds?
- find parent chain FIRST
- identify principal functional grp
- use correct stem (eg meth-, eth-, etc.)
- use di, tri, tetra, etc. if > 1 of same substituent
- number & word separated by -
- use lowest possible number
Define isomers
compounds that hv same molecular formula but diff structural formula
What are the 2 classes of isomerism? give eg for both
- constitutional (structural) isomerism
eg
- chain
- positional
- functional grp - stereoisomerism (same formula, structural formula but diff in 3D space)
eg
- cis-trans
- enantiomerism
Define chain isomer
constitutional isomers w diff arrangement of C chain
eg butane vs 2-methyl propane
Define positional isomer
constitutional isomers w diff pos n of same functional grp on C chain/ring
eg 1-chloropropane vs 2-chloropropane
OR
propan-1-ol vs propan-2-ol
Define functional grp isomer
constitutional isomers w diff functional grp present
eg ethanoic acid vs methyl methanoate
What are criteria for cis-trans isomerism?
A compound will exhibit cis-trans isomerism if:
1. there is restricted rotat n abt a C=C bond due to presence of π bonds
2. there are 2 diff substituent attached to each C atom of C=C bond w restricted rotat n
What is the diff btw cis- & trans- isomer?
- cis-isomer hv 2 identical grp on same side of C=C bond
- trans-isomer hv 2 identical grp on opposite side of C=C bond
Why can we not use condensed formula eg -C3H7 but write as -CH2CH2CH3?
To avoid ambiguity
Which kind of alkene generally do not exhibit cis-trans isomerism? Why?
cyclic alkenes, oni hv cis configurat n
bcos to hv trans config, ring need b distorted (puckered), result in considerable ring strain, lead to instability
What is the formula to predict no of cis-trans isomers?
max no of cis-trans isomers = 2^m
where m>0, m is no of C=C bonds that can exhibit cis-trans isomerism
What are possible explanations to the reason:
- cis-isomer hv higher bp than trans-isomer?
- trans-isomer hv higher mp than cis-isomer?
(all bcos of shape; trans is zig-zag, cis is u shaped)
- cis-isomer hv higher bp than trans-isomer
more energy need to overcome stronger pd-pd attract n btw polar cis-isomer molecules than weaker id-id attract n btw non-polar trans-isomer molecules - (oni applicable if trans-isomer really hv higher mp than cis-isomer; otherwise similar expl n as above)
trans-isomer hv more symmetrical, linear shape, enable closer packing of molecules in solid state. Thus, more extensive imf of attract n formed, require more energy to overcome
Define chiral carbon. How to identify them in cyclic compounds?
a C atom w 4 diff atoms,grp of atoms bonded to it.
- in cyclic compounds, found at 3/4 pt junctions, where both sides of ring from the junction are not identical
What is in a stereochemical formula (showing 3D spatial arrangement of atoms in molecule and molecule shape)?
- ____(bonds on same plane as paper)
- ||||||(bonds behind plane of paper; away from you)
- solid wedge/solid long triangle (bonds out of plane of paper; towards you)
What are the criteria for enantiomerism?
a molecule exhibits enantiomerism/ exists as enantiomers if it hv:
- non-superimposable (not same exact molecule) mirror image
- 1 or more chiral C atom
- no plane of symmetry
What are properties of enantiomers?
- each enantiomer is optically active (can rotate plane-polarised light)
- hv identical chemical properties (except in interact n w another chiral molecule)
- hv identical physical properties EXCEPT in direct n in which they rotate plane-polarised light
- enantiomers rotate light oppositely; (+) enantiomer rotate plane-polarised light clockwise, (-) enantiomer rotate plane-polarised light anti-clockwise
Define racemic mixture. What is one property of such mixture? Why?
- a mixture containing equal amt of each enantiomer
- mixture is optically inactive since no NET rotat n of plane polarised light
bcos both (+), (-) enantiomers rotate plane polarised light thru equal angle but in opp direct n. Hence, rotat n caused by 1 enantiomer is exactly cancelled by rotat n caused by other enantiomer
What is the formula to predict no of enantiomers?
max no = 2^n
where n>0, n is no. of chiral C in molecule
What is the formula to determine TOTAL no of stereoisomers (cis-trans & enantiomers)
max no. = 2^(m+n)
where m, n >0,
m is no of C=C bonds that can exhibit cis-trans isomerism,
n is no of chiral C in molecule
What are biological properties of enantiomers? Explain
- diff stereoisomers exhibit diff biological properties eg in drug action
- most drugs r chiral. Typically, oni 1 of stereoisomers (eg oni 1 of 2 enantiomers) of drug provides desired biological effect. The other mirror image form can b less active & toxic
=> bcos to exert biological action, chiral drug molecule must fit into chiral receptor at some target site in body. Oni 1 of mirror images can fit properly into receptor to exert biological action. The other mirror image is thus inactive or may interact w other unintended receptors, giving rise to undesired side effects
What type of hybridisation in C atom is present if there are:
- 4 σ bonds?
- 3 σ bonds?
- 2 σ bonds?
- 4 σ bonds - sp3 hybridised
- 3 σ bonds - sp2 hybridised
- 2 σ bonds - sp hybridised
(Tip: remember no of σ bonds then minus 1 to get sp…
eg 4 σ bond –> 4-1=3 –> sp3 hybridised)
Explain the formation of following hybrid orbitals, along with shape, bond angle around C:
i. sp3
ii. sp2
iii. sp
i. sp3
- 1 s, 3 p orbitals (s,px,py,pz) r involved in hybridisat n process
- 4 sp3 hybrid orbitals r formed
- tetrahedral, 109.5°
ii. sp2
- 1 s, 2 p orbitals r involved in hybridisat n process
- 3rd p orbitals remain unhybridised
- 3 sp2 hybrid orbitals r formed
- trigonal planar, 120°
iii. sp
- 1 s, 1 p orbitals r involved in hybridisat n process
- 2 other p orbitals remain unhybridised
- 2 sp hybrid orbitals r formed
- linear, 180°
