[Music] hi and welcome back to free science lessons by the end of this video you should be able to describe the structure of halo alkanes you should then be able to describe the physical properties of haloalkanes i should point out that haloalkanes are also called halogenoalkanes but i'll be using the term haloalkanes in this topic okay i'm showing you three haloalkanes here as you can see haloalkanes contain a halogen atom bonded to an alkane in the name the halogen is the prefix followed by the parent alkane and remember that we use numbers to show the positions of functional groups such as the halogen now halo alkenes like this with one halogen atom have the general formula cn h2n plus one x and x is used to represent the halogen haloalkanes can also have more than one halogen atom for example one two dichloroethane and one two three tribromo propane i'm showing you here two bromo one chloropropane when we have different halogens then we list the halogens alphabetically not by position number okay now just like alcohols we can classify haloalkanes as primary secondary and tertiary in primary haloalkanes the halogen is bonded to a carbon atom which is bonded to one other carbon atom in secondary haloalkanes the halogens bonded to a carbon atom which is bonded to two other carbon atoms and in tertiary haloalkanes the halogens bonded to a carbon atom which is bonded to three other carbon atoms okay now a key idea you need to understand is that the carbon to halogen bond is polar and this polarity affects both the physical properties and reactivity of haloalkanes remember that halogen atoms are electronegative this means that the pair of electrons in the covalent bond between the carbon atom and halogen atom is closer to the halogen than the carbon because of this the halogen atom has a slight negative charge and the carbon atom has a slight positive charge now in the next video we'll start looking at how this bond polarity affects the reactivity of the haloalkanes but we're going to finish this video by looking at their physical properties i'm showing you here the boiling point of some haloalkanes and the equivalent alkanes the first idea you need to understand is that haloalkanes have higher boiling points than the equivalent alkanes and this is because of the intermolecular forces alkanes are nonpolar molecules so the intermolecular forces are london forces london forces are relatively weak and require little energy to break so alkanes have relatively low boiling points in halo alkanes we also find london forces however because of the polarity of the carbon to halogen bond we also find permanent dipole dipole-dipole interactions permanent dipole-dipole interactions are stronger than london forces and require more energy to break so this explains why haloalkanes have higher boiling points than equivalent alkanes i'm showing you here the boiling points of four haloalkanes based on ethane as you can see as we go down group seven the boiling point of the haloalkane increases and again this is due to london forces the number of electrons in the halogen atom increases as we go down group seven and london forces are larger when there are more electrons larger london forces require more energy to break and this explains why the boiling point increases as we increase the size of the halogen atom now haloalkanes are insoluble in water and that's because haloalkanes cannot form hydrogen bonds however haloalkanes are soluble in non-polar solvents such as cyclohexane okay so hopefully now you can describe the structure and physical properties of haloalkanes [Music]