Alkanes Flashcards
what is a hydrocarbon?
compounds that only contain hydrogen and carbon
What does saturated mean?
compound which contains only single bonds between carbon atoms
What is the general formula for alkanes?
Cn H2n+2
What is a functional group?
an atom or group of atoms that gives a compound its characteristic and chemical reactivity
What is a homologus series?
a series of compounds that have the same functional group and the same general formula and in which the successive members of the series differ by an extra CH2 unit
What shape are the hydrogens around each carbon in an alkane?
tetrahedral - bond angle 109.5’
What are isomers?
different compounds that have the same molecular formula
What are structural isomers?
compounds that have the same molecular formula but different structural formula
What is a branch in the alkane called and what is its general formula?
akyl group - CnH2n+1
eg: methyl, ethyl
What does it mean if the alkane is substituted?
1 or more of the hydrogens is replaced by a different atom or group of atoms
What is a cycloalkane’s general formula?
CnH2n
Why do alkanes have relatively low boiling points?
C-H bonds are non-polar so they can only form london forces which are the weakest type of intermolecular forces
What increases an alkanes boiling point?
increasing the number of carbon atoms
Why does an alkanes boiling point increase with more carbon atoms?
the size of the atom increases = the surface contact area with other molecules increases
therefore:
more london forces to break and more energy is needed to break them
Why do straight chain alkanes have a higher boiling point than branched isomers?
the straight chain isomer has a larger surface area for contact with other molecules - can form more london forces - more energy needed
What is the density of alkanes like?
all solid and liquid alkanes are less dense than water so they will float on water
Why are alkanes insoluble?
non-polar so cannot form hydrogen bonds
Is combustion of alkanes endothermic or exothermic?
exothermic
What is the expression for the number of products and reactants in the combustion of alkanes?
CnH2n+2 + 3n+1/2 O2 —–> nCO2 + n+1 H2O
What are the products of incomplete combustion of alkanes?
CO or C (soot)
What does alkanes reacting with halogens produce?
haloalkanes
What type of reaction is alkanes reacting with halogens?
substitution
What are the conditions for alkanes reacting with halogens?
room temp under exposure to UV light, does not occur in the dark
What are the observations for alkanes reacting with bromine?
bromine is decolourised - orange –> colourless
steamy, acidic fumes formed = HBr
What does aliphatic mean?
C atoms are joined to each other in unbranched or branched chains or non-aromatic rings
What does alicyclic mean?
C atoms are joined to each other in a ring structure with or without branches
What does aromatic mean?
some or all of the C atoms are found in a bezene ring
What is a reaction mechanism?
a description of the key events on the pathway that leads reactants to come together and react to produce the products
What is a radical?
an atom or group of atoms which has an unpaired electron, they are often highly reactive
What happens during the initiation of alkanes with halogens?
starts the reaction, brought about bu UV light which provides the energy needed to break a halogen molecule apart to produce its radical
the bond breaking is by homolytic fission
What is homolytic fission?
a process in which a covalent bond breaks so that one of the electrons that one of the electrons that made up the bond stays with one of the atoms and the other electron stays with the other atom
What happens during the propagation stage in the reaction of alkanes with halogens?
x2 reactions:
1. a highly reactive halogen collides with a methane molecule and extracts a H atom from it, forming a molecule of acid and a methyl radical - CH3-
2. the reactive methyl radical collides with a halogen molecule and extracts a halogen molecule from it, forming a molecule of halogenmethane and a halogen radical
the two reactions can repeat themselves = a chain reaction
What is the termination stage for the reaction with alkanes and halogens?
if two radicals collide with each other, they combine to form a molecule which now has no unpaired electrons this stops the reaction
What is heterolytic fissison?
when a covalent bond breaks, one of the bonded atoms takes both of electrons from the bond
