alkanes and alkenes

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  • Created by: Bassi123
  • Created on: 23-01-18 10:06
WHAT ARE ALKANES ?
CnHn2+2 - they are saturated hydrocarbons, containing only carbon and hyrdrogen atoms joined together by single bonds.
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BONDING IN ALKANES - sigma
each carbon atom is joined to four other atoms by single covalent bonds. These types of covalent bond are called a sigma bond
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WHAT IS A COVALENT BOND ?
The electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms
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WHAT IS A SIGMA BOND?
A sigma bond is the result of the overlap of two orbitals, one from each bonding atom. Each overlapping orbital contains one electron so the sigma bond has two e- that are shared between the bonding atoms. Each C atom has 4 sigma bonds C-C, C-H
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WHAT ARE THE SHAPES OF ALKANES?
each C atom is surrounded by 4 e- pairs in 4 sigma bonds. Repulsion between these e- pairs result in a 3D tetrahedral arrangement around each carbon atom. Bond angle = 109.5
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ALKANES CAN ROTATE FREELY
The sigma bond acts as an axes around which the atoms can rotate freely, so these shapes are not rigid.
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BP OF ALKANES
bp increases as their carbon chain length increases. LF act between molecules that are in close surface contact. As chain length increases, the molecules have a larger surface area, so more surface contact. The LF between the molecules is greater.
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EFFECT OF BRANCHING ON ALKANES
Branched isomers have a lower bp. There are fewer points of contact between molecules of the branched alkanes, giving fewer LF. Branches get in the way and prevent branched molecules getting as close together as straight chained molecules, lower IMF
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FREE RADICAL SUBSTITUTION
mechanism - 3 steps - Initiation, propagation, termination. Need UV radiation - provides the initial energy for a reaction to take place. Photodissociation
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FREE RADICAL SUBSTITUTION - INITIATION
CL2 -> 2CL• (covalent bond in chlorine molecule is broken by homolytic fission, each Cl atom takes 1 e- from the pair, forming highly reactive bromine radicals.
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FREE RADICAL SUBSTITUTION - PROPAGATION (METHANE)
CH4 + Cl• -> •CH3 + HCl - step 1 •CH3 + Cl2 -> CH3Cl + Cl• - step 2
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FREE RADICAL SUBSTITUTION - TERMINATION
Step 1 - Cl• + Cl• -> 2Cl2 , Step 2- •CH3 + •CH3 -> C2H6 , Step 3 - •CH3 + Cl• -> CH3Cl
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LIMITATIONS OF RADICAL SUBSTITUTION
Further subsititution - can continue until all hydrogen atoms have been substituted. Get a result of different organic products
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WHAT IS A RADICAL?
A species with an unpaired electron
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WHAT IS AN ALKENE
Unsaturated hydrocarbons - contain at least 1 double C=C
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FORMULA FOR ALKENES
Aliphatic alkenes general formual = CnH2n ( whilst branched alkenes obey the general formula, cyclic alkenes and alkenes with more than 1 double bond do not.
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NATURE OF DOUBLE BOND 1
For each C atom of the double bond, 3 of the 4 e- are used in the 3 sigma bonds, 1 to the other C atom of the double bond and the other 2 e- to 2 other atoms ( C-H)
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NATURE OF DOUBLE BOND 2 - pi bond
Leave one electron on each C atom of the double bond not involved in the sigma bonds. This e- is in a P-orbital. The pi bond is formed by the sideways overlap of 2 p-orbitals, one from each C atome of the double bond
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NATURE OF DOUBLE BOND 3 - pi bond
Each C atom contributes 1 e- to the e- pair in the pi bond. The pi- electron density is conc above and below the line joining the nuclei of the bonding atoms
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LACK OF ROTATION IN ALKENES
The pi bond locks the 2 C atoms in position and prevents them from rotating around the double bond.
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SHAPE AROUND A DOUBLE BOND
The shape around each C atom in the double bond is TRIGONAL PLANAR
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WHY IS THE SHAPE OF ALKENES TRIGONAL PLANAR
There are 3 regions of electron density around each of the C atoms. The 3 regions repel each other as far as possible so bond around each C atom is 120. All atoms in same planar
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STEREOISOMERS - DEFINITION
same structural formula but different arrangement of atoms in space.
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WHAT ARE THE 2 TYPES OF STEREOISOMERS?
E/Z isomerism and Optical isomerism
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WHERE DOES E/Z ISOMERISM OCCUR?
only occurs in compounds with a C=C double bond
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WHERE DOES OPTICAL ISOMERISM OCCUR?
can occur in a much wider range of compunds, including alkanes with no functional groups.
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E/Z ISOMERISM
stereoisomerism around double bonds arises because rotation about the double bond is restricted and the groups attached to each carbon atom are fixed relative to each other - rigidity due to the position of the pi-bonds e- density above and below
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WHAT ARE THE 2 CONDITIONS FOR E/Z ISOMERISM
1) A C=C double bond 2) different groups attached to each carbon atom of teh double bond
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WHAT IS CIS-TRANS ISOMERISM?
Molecules must have C=C double bond and each C in the double bond must be attached to 2 different groups, as for all E/Z isomers.
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CIS ISOMER
Has the H atoms on each carbon in the double bond on the SAME side of the molecule
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TRANS ISOMER
Has H atoms diagonally OPPOSITE each other
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CAHN- INGOLD- PRELOG NOMENCLATURE
The cis-trans of naming acn only be used when each C atom in the double bond is attached to a single H atom.
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CAHN- INGOLD- PRELOG rules
1) if groups of higher priority are on the same side of double bond, the compound is Z isomer 2) if groups of higher priority are diagonally placed across the double bond - E isomer
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Other cards in this set

Card 2

Front

BONDING IN ALKANES - sigma

Back

each carbon atom is joined to four other atoms by single covalent bonds. These types of covalent bond are called a sigma bond

Card 3

Front

WHAT IS A COVALENT BOND ?

Back

Preview of the front of card 3

Card 4

Front

WHAT IS A SIGMA BOND?

Back

Preview of the front of card 4

Card 5

Front

WHAT ARE THE SHAPES OF ALKANES?

Back

Preview of the front of card 5
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Georgie1310

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what does LF stand for in these cards?

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