Organic CHEM Flashcards
(41 cards)
The physical properties of Alkanes
Properties that can be observed or measured without changing the composition of the substance
their solubility in water
their boiling point
Th boiling points of straight chain alkanes
Increases as the chain length increases
Boiling points of Alkanes
Boiling involves converting a liquid to a gas and requires energy (heat) to overcome the intermolecular interactions between molecules (Van-der Waals) attractions between molecules of alkanes
Non-Covalent intercations
do not involve sharing of valence electrons
these interactions are due to electrostatic interactions (attraction between opposite charges)
there are 2 categories: attraction between permanent and attraction between temporary partial charges
Attraction between permanent charges
Hydrogen bonds, salt ridges/ionic bonds, dipole-dipole interactions, ion-dipole interactions and dative bonds
Attraction between temporary partial charges
London dispersion forces (Van der Waals)
Hydrogen bonding
IUPAC definition:
hydrogen bonding occurs when an electron deficient hydrogen that is bonded to an atom, has an attractive interaction with another electron rich region either within the same or another molecular entity
for our purposes:
A hydrogen bond is the interaction of a hydrogen atom bonded to highly electronegative atoms (N,O,F) and the non-bonding electron pairs on other highly electronegative atoms
Salt bridges/ionic bonding
the name given by biochemists to the ionic bonds that form between the positively and negatively charged groups of amino acid side chains found in proteins
Dipole-dipole interactions
A polar covalent bond can also be called a permanent Dipole
when two permanent dipoles are in close proximity (such as carbonyl) , they will align themselves. The partially positive end of one dipole is attracted to the partially negative end of another.
Van der Waals forces (London dispersion forces)
Due to attractive interactions between temporary dipoles
all molecules experience these in addition to other types of interactions
they are the weakest type of intermolecular interaction and the only act over short distances
At any instant, electron movement in a molecule can create a temporary dipole with partial positive and negative charges
The temporary dipole of the first molecule
indices a temporary dipole in a molecule close by
there is a weak attractive force between oppositely charged parts of the molecule. Although the attractive interaction is very weak, in bulk media there can be lots of them so they are still important attractive forces
Important points to remember about van Der Waals forces
- They are bright about by a temporary dipole that induces another temporary dipole in a nearby molecule
- The are weaker than all other non-covalent attractive forces discussed and so relatively little energy is required to overcome them
- They act over very short distances compared to the other interactions we have discussed
short range - therefore they are more effective when the molecules are able to get closer together
Why the boiling point increases as the chain length increases
-The electrons in an alkane molecule are always moving
-When they become unevenly distributed over the surface of the molecule some regions of the surface have a slightly positive and others a slightly negative charge
-we say that the molecule is temporarily polarised
-the temporary polarisation of an alkane molecule induces the temporary polarisation in nearby alkane molecules
-then these molecules become weakly attracted to each other
Van der waals
operate over short distances and between the surfaces of the molecules, so molecules need to be close together to be attracted to one another
For straight chain alkanes
the larger the molecule (the greater the molecular weight of the alkane or the more carbon atoms in the chain) the higher the boiling point (as van der waal forces increase)
BUT for branched alkanes
The boiling points of alkanes with the same molecular weight decreases as you increase the number of branches.
In straight chain pentane
the surface area between the molecules is larger (molecules can get very close to each other) and so Va der waals attractions between them are greater than for spherical 2,2-dimethylpropane. Therefore, pentane has a higher boiling point
Chemical changes causes
a change in the bonds of/between molecules, atoms or ions
Physical changes causes
no change in bonding
A new substance or substances is/are formed by
a chemical reaction
The chemical reactions of alkanes
C-C bonds and C-H bonds are single (saturated hydrocarbons)
non-polar covalent
therefore, alkanes are relatively inert (unreactive)
they do undergo two main types of chemical reactions
Combustion
A highly exothermic reaction (heat given out)
basis for use of hydrocarbons to provide heat and power
combustion of hydrocarbons produces CO2 (greenhouse gases)
incomplete combustion occurs if not enough oxygen present in this case carbon monoxide and carbon (soot) are formed as products of the reaction
inert
unreactive
Halogenation
-A source of light or heat are required for the reaction to proceed
-example (The chlorination of methane)
